Bring gitdb submodule back to a version that supports python 2.x

Byron · Byron · commit 8defbd88bf06 · 2020-02-17T12:05:51.000+08:00
This should fix tests on travis.
diff --git a/git/ext/gitdb b/git/ext/gitdb
@@ -1 +1 @@
-Subproject commit 5dd0f302f101e66d9d70a3b17ce0f379b4db214b
+Subproject commit c0fd43b5ff8c356fcf9cdebbbbd1803a502b4651
