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>

main.py

@@ -789,7 +789,7 @@ def init_http():
             mgr.add_password(p[1], "https://%s/" % host, p[0], p[2])
     except netrc.NetrcParseError:
         pass
-    except IOError:
+    except OSError:
         pass
     handlers.append(_BasicAuthHandler(mgr))
     handlers.append(_DigestAuthHandler(mgr))