revert unnecessary change to mypy_test from previous commit

tests/mypy_test.py

@@ -22,7 +22,7 @@ parser = argparse.ArgumentParser(description="Test runner for typeshed. "
 parser.add_argument('-v', '--verbose', action='count', default=0, help="More output")
 parser.add_argument('-n', '--dry-run', action='store_true', help="Don't actually run mypy")
 parser.add_argument('-x', '--exclude', type=str, nargs='*', help="Exclude pattern")
-parser.add_argument('-p', '--python-version', type=str, action='append',
+parser.add_argument('-p', '--python-version', type=str, nargs='*',
                     help="These versions only (major[.minor])")
 parser.add_argument('filter', type=str, nargs='*', help="Include pattern (default all)")