convert metadata.Build to a subclass of dict

Like with the App class in the commit before, this makes it a lot
easier to work with this data when converting between the internal
formats and external formats like YAML, JSON, MsgPack, protobuf, etc.

The one unfortunate thing here is Build.update. It becomes
dict.update(), which is a method not an attribute.
build.get('update') or build['update'] could be used, but that would
be oddly inconsistent. So instead the field is renamed to
'androidupdate', except for in the .txt v0 metadata files. This better
describes what field does anyway, since it runs `android update`.

Build.update is only referenced in two places right next to each other
for the ant builds, so this change still seems worthwhile.

fdroidserver/metadata.py

@@ -25,7 +25,6 @@ import cgi
 import logging
 import textwrap
 import io
-import pprint
 
 import yaml
 # use libyaml if it is available
@@ -242,9 +241,10 @@ build_flags_order = [
 build_flags = set(build_flags_order + ['version', 'vercode'])
 
 
-class Build():
+class Build(dict):
 
     def __init__(self, copydict=None):
+        super().__init__()
         self.disable = False
         self.commit = None
         self.subdir = None
@@ -263,7 +263,7 @@ class Build():
         self.rm = []
         self.extlibs = []
         self.prebuild = ''
-        self.update = []
+        self.androidupdate = []
         self.target = None
         self.scanignore = []
         self.scandelete = []
@@ -274,45 +274,28 @@ class Build():
         self.gradleprops = []
         self.antcommands = []
         self.novcheck = False
-
-        self._modified = set()
-
         if copydict:
-            for k, v in copydict.items():
-                self.set_flag(k, v)
+            super().__init__(copydict)
+            return
 
-    def __str__(self):
-        return pprint.pformat(self.__dict__)
-
-    def __repr__(self):
-        return self.__str__()
-
-    def get_flag(self, f):
-        if f not in build_flags:
-            warn_or_exception('Unrecognised build flag: ' + f)
-        return getattr(self, f)
-
-    def set_flag(self, f, v):
-        if f == 'versionName':
-            f = 'version'
-        if f == 'versionCode':
-            f = 'vercode'
-        if f not in build_flags:
-            warn_or_exception('Unrecognised build flag: ' + f)
-        self.__dict__[f] = v
-        self._modified.add(f)
-
-    def append_flag(self, f, v):
-        if f not in build_flags:
-            warn_or_exception('Unrecognised build flag: ' + f)
-        if f not in self.__dict__:
-            self.__dict__[f] = [v]
+    def __getattr__(self, name):
+        if name in self:
+            return self[name]
         else:
-            self.__dict__[f].append(v)
+            raise AttributeError("No such attribute: " + name)
+
+    def __setattr__(self, name, value):
+        self[name] = value
+
+    def __delattr__(self, name):
+        if name in self:
+            del self[name]
+        else:
+            raise AttributeError("No such attribute: " + name)
 
     def build_method(self):
         for f in ['maven', 'gradle', 'kivy']:
-            if self.get_flag(f):
+            if self.get(f):
                 return f
         if self.output:
             return 'raw'
@@ -323,7 +306,7 @@ class Build():
         if self.output:
             return 'raw'
         for f in ['maven', 'gradle', 'kivy']:
-            if self.get_flag(f):
+            if self.get(f):
                 return f
         return 'ant'
 
@@ -336,10 +319,6 @@ class Build():
             return ''
         return paths[version]
 
-    def update_flags(self, d):
-        for f, v in d.items():
-            self.set_flag(f, v)
-
 
 flagtypes = {
     'extlibs': TYPE_LIST,
@@ -348,7 +327,7 @@ flagtypes = {
     'rm': TYPE_LIST,
     'buildjni': TYPE_LIST,
     'preassemble': TYPE_LIST,
-    'update': TYPE_LIST,
+    'androidupdate': TYPE_LIST,
     'scanignore': TYPE_LIST,
     'scandelete': TYPE_LIST,
     'gradle': TYPE_LIST,
@@ -982,14 +961,16 @@ def parse_txt_metadata(mf, app):
 
         pk, pv = bv
         pk = pk.lstrip()
+        if pk == 'update':
+            pk = 'androidupdate'  # avoid conflicting with Build(dict).update()
         t = flagtype(pk)
         if t == TYPE_LIST:
             pv = split_list_values(pv)
-            build.set_flag(pk, pv)
+            build[pk] = pv
         elif t == TYPE_STRING or t == TYPE_SCRIPT:
-            build.set_flag(pk, pv)
+            build[pk] = pv
         elif t == TYPE_BOOL:
-            build.set_flag(pk, _decode_bool(pv))
+            build[pk] = _decode_bool(pv)
 
     def parse_buildline(lines):
         v = "".join(lines)
@@ -1231,7 +1212,7 @@ def write_plaintext_metadata(mf, app, w_comment, w_field, w_build):
         if build.version == "Ignore":
             continue
 
-        w_comments('build:' + build.vercode)
+        w_comments('build:%s' % build.vercode)
         w_build(build)
         mf.write('\n')
 
@@ -1276,11 +1257,13 @@ def write_txt(mf, app):
         mf.write("Build:%s,%s\n" % (build.version, build.vercode))
 
         for f in build_flags_order:
-            v = build.get_flag(f)
+            v = build.get(f)
             if not v:
                 continue
 
             t = flagtype(f)
+            if f == 'androidupdate':
+                f == 'update'  # avoid conflicting with Build(dict).update()
             mf.write('    %s=' % f)
             if t == TYPE_STRING:
                 mf.write(v)
@@ -1357,7 +1340,7 @@ def write_yaml(mf, app):
         w_field('versionName', build.version, '  - ', TYPE_STRING)
         w_field('versionCode', build.vercode, '    ', TYPE_STRING)
         for f in build_flags_order:
-            v = build.get_flag(f)
+            v = build.get(f)
             if not v:
                 continue