Don't attempt to fixup VCS direct URLs for older python versions

The script does not work with python 2.7 and pip's legacy resolver
that works with these older pythons does not need this workaround.

src/pip_deepfreeze/pip.py

@@ -23,7 +23,7 @@
     parse as parse_req_file,
 )
 from .req_parser import get_req_name
-from .sanity import get_pip_version
+from .sanity import _get_env_info, get_pip_version
 from .utils import (
     check_call,
     check_output,
@@ -269,6 +269,9 @@ def pip_uninstall(python: str, requirements: Iterable[str]) -> None:
 
 
 def pip_fixup_vcs_direct_urls(python: str) -> None:
+    if Version(_get_env_info(python)["python_version"]) < Version("3.6"):
+        # Not supported, and not needed with pip's legacy resolver.
+        return
     # This will become unnecessary when pip caches metadata or
     # caches wheels that are built from a git ref that is not a commit
     # (it could cache it under a modified link where the git ref is replaced