Fix Python SyntaxError

Python has started warning about invalid escape sequences in strings.
This fixes the warning by escaping the backslash in question. It seems
Python was already (correctly) printing the literal backslash despite
the warning, so no behavioral change.

Bug: None
Change-Id: I2d5d8d56b989d9a1bb87aba69af0ebac0209a7a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6299490
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Brian Ryner <bryner@google.com>

tests/git_cl_test.py

@@ -981,7 +981,7 @@ class TestGitCl(unittest.TestCase):
                     ref_suffix
                 ] + (push_opts if push_opts else []), ),
                 (('remote:\n'
-                  'remote: Processing changes: (\)\n'
+                  'remote: Processing changes: (\\)\n'
                   'remote: Processing changes: (|)\n'
                   'remote: Processing changes: (/)\n'
                   'remote: Processing changes: (-)\n'