encode filenames as bytes to handle all locale setups

This was failing on environments that did not have any LANG or LC_* locale
variables set.  This is a valid setup, and is common in headless setups, so
it needs to be handled.

This also adds a new pass of the test suite without the locale env vars set
so that this situation is also tests on gitlab-ci, not only gpjenkins.

The error this caused was:
UnicodeEncodeError: 'ascii' codec can't encode characters in position 6-18: ordinal not in range(128)

fdroidserver/common.py

@@ -1700,7 +1700,8 @@ class KnownApks:
 
 def get_file_extension(filename):
     """get the normalized file extension, can be blank string but never None"""
-
+    if isinstance(filename, bytes):
+        filename = filename.decode('utf-8')
     return os.path.splitext(filename)[1].lower()[1:]
 
 
@@ -2333,15 +2334,17 @@ def get_per_app_repos():
 
 def is_repo_file(filename):
     '''Whether the file in a repo is a build product to be delivered to users'''
+    if isinstance(filename, str):
+        filename = filename.encode('utf-8', errors="surrogateescape")
     return os.path.isfile(filename) \
-        and not filename.endswith('.asc') \
-        and not filename.endswith('.sig') \
+        and not filename.endswith(b'.asc') \
+        and not filename.endswith(b'.sig') \
         and os.path.basename(filename) not in [
-            'index.jar',
-            'index_unsigned.jar',
-            'index.xml',
-            'index.html',
-            'index-v1.jar',
-            'index-v1.json',
-            'categories.txt',
+            b'index.jar',
+            b'index_unsigned.jar',
+            b'index.xml',
+            b'index.html',
+            b'index-v1.jar',
+            b'index-v1.json',
+            b'categories.txt',
         ]