fix: replace deprecated codecs.open with built-in open (#128)

Python 3.14 emits a DeprecationWarning for codecs.open(), which gitdb
hits inside ReferenceDB._update_dbs_from_ref_file:

    DeprecationWarning: codecs.open() is deprecated. Use open() instead.

The built-in open() has supported the encoding kwarg since Python 3.0
and the call site already passes encoding="utf-8", so the replacement
is byte-for-byte equivalent on every supported Python version.

Dropped the now-unused codecs import.

Verified the change with the existing test_ref.py suite.

Closes #128

Author: mvanhorn

gitdb/db/ref.py

@@ -2,7 +2,6 @@
 #
 # This module is part of GitDB and is released under
 # the New BSD License: https://opensource.org/license/bsd-3-clause/
-import codecs
 from gitdb.db.base import (
     CompoundDB,
 )
@@ -42,7 +41,7 @@ def _update_dbs_from_ref_file(self):
         # try to get as many as possible, don't fail if some are unavailable
         ref_paths = list()
         try:
-            with codecs.open(self._ref_file, 'r', encoding="utf-8") as f:
+            with open(self._ref_file, 'r', encoding="utf-8") as f:
                 ref_paths = [l.strip() for l in f]
         except OSError:
             pass