In Nixpkgs 20.03, Mercurial fails if PYTHONPATH is set:

$ hg Traceback (most recent call last): File "/nix/store/q7s856v6nw4dffdrm9k3w38qs35i8kr3-mercurial-5.2.2/bin/..hg-wrapped-wrapped", line 37, in <module> dispatch.run() File "/nix/store/bffdy7q3wi3qinflnvbdkigqj39zzynd-python3-3.7.6/lib/python3.7/importlib/util.py", line 245, in getattribute self.spec.loader.exec_module(self) File "<frozen importlib._bootstrap_external>", line 728, in exec_module File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed File "/nix/store/q7s856v6nw4dffdrm9k3w38qs35i8kr3-mercurial-5.2.2/lib/python3.7/site-packages/mercurial/dispatch.py", line 10, in <module> import difflib File "/nix/store/bffdy7q3wi3qinflnvbdkigqj39zzynd-python3-3.7.6/lib/python3.7/difflib.py", line 1084, in <module> import re File "/nix/store/bffdy7q3wi3qinflnvbdkigqj39zzynd-python3-3.7.6/lib/python3.7/re.py", line 143, in <module> class RegexFlag(enum.IntFlag): AttributeError: module 'enum' has no attribute 'IntFlag'