Avoid invalid escape sequence errors on Windows.

Change-Id: I3ed566fb37f235190b5fe91dfd07f575173a6aca
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62711
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>

python/setup.py

@@ -155,7 +155,7 @@ cc_import(
 cc_library(
     name = "libraries",
 )
-""".format(interpreter_path=sys.executable,
+""".format(interpreter_path=sys.executable.replace('\\', '/'),
            major=sys.version_info.major,
            minor=sys.version_info.minor))