always open Android source files as UTF-8

Android Studio recommends "you use UTF-8 encoding whenever possible",
so this code assumes the files use UTF-8.  UTF-8 is also the default
encoding on GNU/Linux and macOS.
https://sites.google.com/a/android.com/tools/knownissues/encoding

Windows will probably default to UTF16, since that's the native
encoding for files.  So forcing things to use UTF-8 should help
compatibility.

tests/build.TestCase

@@ -339,12 +339,12 @@ class BuildTest(unittest.TestCase):
 
         os.mkdir('build')
         os.mkdir('build/reports')
-        with open('build.gradle', 'w') as fp:
+        with open('build.gradle', 'w', encoding='utf-8') as fp:
             fp.write('// placeholder')
 
         os.mkdir('bin')
         os.mkdir('gen')
-        with open('build.xml', 'w') as fp:
+        with open('build.xml', 'w', encoding='utf-8') as fp:
             fp.write(
                 textwrap.dedent(
                     """<?xml version="1.0" encoding="UTF-8" standalone="no"?>

tests/common.TestCase

@@ -315,14 +315,15 @@ class CommonTest(unittest.TestCase):
         fdroidserver.common.prepare_source(FakeVcs(), app, build,
                                            fdroidclient_testdir, fdroidclient_testdir, fdroidclient_testdir)
 
-        with open(os.path.join(fdroidclient_testdir, 'build.gradle'), 'r') as f:
-            filedata = f.read()
+        fdroidclient_testdir = Path(fdroidclient_testdir)
+        build_gradle = fdroidclient_testdir / 'build.gradle'
+        filedata = build_gradle.read_text(encoding='utf-8')
         self.assertIsNotNone(
             re.search(r"\s+compileSdkVersion %s\s+" % testint, filedata)
         )
 
-        with open(os.path.join(fdroidclient_testdir, 'AndroidManifest.xml')) as f:
-            filedata = f.read()
+        androidmanifest_xml = fdroidclient_testdir / 'AndroidManifest.xml'
+        filedata = androidmanifest_xml.read_text(encoding='utf-8')
         self.assertIsNone(re.search('android:debuggable', filedata))
         self.assertIsNotNone(
             re.search('android:versionName="%s"' % build.versionName, filedata)
@@ -342,10 +343,10 @@ class CommonTest(unittest.TestCase):
         )
 
         subdir = 'baz/bar'
-        subdir_path = os.path.join(app_build_dir, subdir)
-        os.makedirs(subdir_path)
-        with open(os.path.join(subdir_path, 'build.gradle'), 'w') as fp:
-            fp.write('// just a test placeholder')
+        subdir_path = Path(app_build_dir) / subdir
+        subdir_path.mkdir(parents=True, exist_ok=True)
+        build_gradle = subdir_path / 'build.gradle'
+        build_gradle.write_text('// just a test placeholder', encoding='utf-8')
 
         config = dict()
         fdroidserver.common.fill_config_defaults(config)
@@ -921,7 +922,7 @@ class CommonTest(unittest.TestCase):
         self.assertNotEqual(0, len(files))
         for f in files:
             appid, versionCode = os.path.splitext(os.path.basename(f))[0][12:].split('_')
-            with open(f) as fp:
+            with open(f, encoding='utf-8') as fp:
                 m = fdroidserver.common.APK_ID_TRIPLET_REGEX.match(fp.read())
                 if m:
                     self.assertEqual(appid, m.group(1))
@@ -962,6 +963,7 @@ class CommonTest(unittest.TestCase):
 
     def test_get_sdkversions_androguard(self):
         """This is a sanity test that androguard isn't broken"""
+
         def get_minSdkVersion(apkfile):
             apk = fdroidserver.common._get_androguard_APK(apkfile)
             return fdroidserver.common.get_min_sdk_version(apk)
@@ -1852,9 +1854,9 @@ class CommonTest(unittest.TestCase):
         self.assertEqual([],
                          data['fdroiddata']['untrackedFiles'])
         dirtyfile = 'dirtyfile'
-        with open(dirtyfile, 'w') as fp:
+        with open(dirtyfile, 'w', encoding='utf-8') as fp:
             fp.write('this is just a test')
-        with open(file_in_git, 'a') as fp:
+        with open(file_in_git, 'a', encoding='utf-8') as fp:
             fp.write('\nappend some stuff')
         self.assertEqual([],
                          data['fdroiddata']['modifiedFiles'])

tests/scanner.TestCase

@@ -76,7 +76,7 @@ class ScannerTest(unittest.TestCase):
             build = fdroidserver.metadata.Build()
             build.gradle = [flavor]
             regexs = fdroidserver.scanner.get_gradle_compile_commands(build)
-            with open(f) as fp:
+            with open(f, encoding='utf-8') as fp:
                 for line in fp.readlines():
                     for regex in regexs:
                         m = regex.match(line)
@@ -93,7 +93,7 @@ class ScannerTest(unittest.TestCase):
         fdroidserver.scanner.config = None
         fdroidserver.scanner.options = mock.Mock()
         fdroidserver.scanner.options.json = True
-        with open('build.gradle', 'w') as fp:
+        with open('build.gradle', 'w', encoding='utf-8') as fp:
             fp.write(
                 textwrap.dedent(
                     """
@@ -233,7 +233,7 @@ class ScannerTest(unittest.TestCase):
                 fp.write('placeholder')
             self.assertTrue(os.path.exists(f))
 
-        with open('build.xml', 'w') as fp:
+        with open('build.xml', 'w', encoding='utf-8') as fp:
             fp.write(
                 textwrap.dedent(
                     """<?xml version="1.0" encoding="UTF-8" standalone="no"?>
@@ -288,7 +288,7 @@ class ScannerTest(unittest.TestCase):
         fdroidserver.scanner.options = mock.Mock()
         build = fdroidserver.metadata.Build()
         build.scandelete = ['build.gradle']
-        with open('build.gradle', 'w') as fp:
+        with open('build.gradle', 'w', encoding='utf-8') as fp:
             fp.write(
                 textwrap.dedent(
                     """