SyntaxWarning when running 'git-webkit pr'

https://bugs.webkit.org/show_bug.cgi?id=260729

Reviewed by Jonathan Bedard.

The backslashes here are python string escapes, but they were intended
to be regex escapes. Each \ would need to be replaced with \\ for them
to function as regex escapes. We can use raw string notation to avoid
this problem.

* Tools/Scripts/webkitpy/common/checkout/checkout.py:
(Checkout):

Canonical link: https://commits.webkit.org/267558@main

Tools/Scripts/webkitpy/common/checkout/checkout.py

@@ -49,8 +49,8 @@
 # FIXME: Move a bunch of ChangeLog-specific processing from SCM to this object.
 # NOTE: All paths returned from this class should be absolute.
 class Checkout(object):
-    COMMIT_SUBJECT_RE = re.compile(b'Subject: \[PATCH ?(\d+\/\d+)?] (.+)')
-    FILTER_BRANCH_PROGRAM = '''import re
+    COMMIT_SUBJECT_RE = re.compile(br'Subject: \[PATCH ?(\d+\/\d+)?] (.+)')
+    FILTER_BRANCH_PROGRAM = r'''import re
 import sys
 
 lines = [l for l in sys.stdin]