Add compatibility for Python 3.6: follow-up

This is a follow-up on !132 (merged) which fixes some stuff that I missed.

src/mpsd_software_manager/utils/filesystem_utils.py

@@ -129,9 +129,9 @@ def clone_repo(
                     ["git", "branch", "-a"], check=True, capture_output=True
                 )
                 branches_list = branches_result.stdout.decode().split("\n")
-                # strip off 'remotes/origin' (needs Python 3.9):
+                removeprefix = lambda s, p: s[len(p) :] if s.startswith(p) else s  # noqa
                 branches_list = [
-                    b.strip().removeprefix("remotes/origin/") for b in branches_list
+                    removeprefix(b.strip(), "remotes/origin/") for b in branches_list
                 ]
                 msg += f"Available branches are {branches_list}"
                 logging.error(msg)