cleanup: Update codebase to expect Python 3.6

- Bump minimum version to Python 3.6.
- Use f-strings in a lot of places.

Change-Id: I2aa70197230fcec2eff8e7c8eb754f20c08075bb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389034
Tested-by: Jason R. Coombs <jaraco@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Jason R. Coombs <jaraco@google.com>

platform_utils.py

@@ -57,8 +57,8 @@ def _validate_winpath(path):
     if _winpath_is_valid(path):
         return path
     raise ValueError(
-        'Path "{}" must be a relative path or an absolute '
-        "path starting with a drive letter".format(path)
+        f'Path "{path}" must be a relative path or an absolute '
+        "path starting with a drive letter"
     )