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_superproject.py

@@ -381,7 +381,7 @@ class Superproject:
         try:
             with open(manifest_path, "w", encoding="utf-8") as fp:
                 fp.write(manifest_str)
-        except IOError as e:
+        except OSError as e:
             self._LogError("cannot write manifest to : {} {}", manifest_path, e)
             return None
         return manifest_path