repo: reexec into Python 3 under Windows

Hopefully enough issues should be resolved now that we can start
forcing Windows users into Python 3 too.

Change-Id: Ic4aad6a0b35ffec7d1372e3da6fca11a2b6fde0b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255353
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

repo

@@ -105,9 +105,7 @@ def check_python_version():
 
 
 if __name__ == '__main__':
-  # TODO(vapier): Enable this on Windows once we have Python 3 issues fixed.
-  if platform.system() != 'Windows':
-    check_python_version()
+  check_python_version()
 
 
 # repo default configuration
@@ -134,7 +132,7 @@ if not REPO_REV:
 # limitations under the License.
 
 # increment this whenever we make important changes to this script
-VERSION = (2, 3)
+VERSION = (2, 4)
 
 # increment this if the MAINTAINER_KEYS block is modified
 KEYRING_VERSION = (2, 3)