use specified encoding when reading extension metadata

fixes #351

src/gpodder/extensions.py

@@ -233,7 +233,8 @@ class ExtensionContainer(object):
         if not filename or not os.path.exists(filename):
             return {}
 
-        extension_py = open(filename).read()
+        encoding = util.guess_encoding(filename)
+        extension_py = open(filename, "r", encoding=encoding).read()
         metadata = dict(re.findall("__([a-z_]+)__ = '([^']+)'", extension_py))
 
         # Support for using gpodder.gettext() as _ to localize text

src/gpodder/util.py

@@ -1887,3 +1887,34 @@ def delete_empty_folders(top):
             if not os.listdir(dirname):
                 os.rmdir(dirname)
 
+
+def guess_encoding(filename):
+    """
+    read filename encoding as defined in PEP 263
+    - BOM marker => utf-8
+    - coding: xxx comment in first 2 lines
+    - else return None
+    >>> guess_encoding("not.there")
+    >>> guess_encoding("setup.py")
+    >>> guess_encoding("share/gpodder/extensions/mpris-listener.py")
+    'utf-8'
+    """
+    def re_encoding(line):
+        m = re.match(b"""^[ \t\v]*#.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+)""", line)
+        if m:
+            return m.group(1).decode()
+        else:
+            return None
+
+    if not filename or not os.path.exists(filename):
+        return None
+
+    with open(filename, "rb") as f:
+        fst = f.readline()
+        if fst[:3] == b"\xef\xbb\xbf":
+            return "utf-8"
+        encoding = re_encoding(fst)
+        if not encoding:
+            snd = f.readline()
+            encoding = re_encoding(snd)
+    return encoding