cleanup: convert exceptions to OSError

In Python 3, these exceptions were merged into OSError, so switch
everything over to that.

Change-Id: If876a28b692de5aa5c62a3bdc8c000793ce52c63
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/390376
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

git_refs.py

@@ -107,8 +107,6 @@ class GitRefs:
         try:
             fd = open(path)
             mtime = os.path.getmtime(path)
-        except IOError:
-            return
         except OSError:
             return
         try: