pythonPackages.lmdb: fix build

This fixes:

pythonCatchConflictsPhase
Found duplicated packages in closure for dependency 'lmdb':
  lmdb 0.97 (/build/lmdb-0.97)
  lmdb 0.97 (/nix/store/js0iimri6y9yqgfc111jzp3mrv5ic9cj-python3.7-lmdb-0.97/lib/python3.7/site-packages)

Package duplicates found in closure, see above. Usually this happens if two packages depend on different version of the same dependency.
builder for '/nix/store/9bcn2m3r5v8slmpj31hxw05j906qgl5l-python3.7-lmdb-0.97.drv' failed with exit code 1

This was probably broken by https://github.com/NixOS/nixpkgs/commit/f7e28bf5d8181926e600a222cb70180519d09726