$ nix-shell -I nixpkgs=/home/genesis/devel/nixpkgs
Fatal Python error: initsite: Failed to import the site module
Traceback (most recent call last):
  File "/nix/store/hm6iw7j16i1r7ppvfagsyzzrb9zjbq7x-python2.7-setuptools-41.0.1/lib/python2.7/site-packages/site.py", line 73, in <module>
    __boot()
  File "/nix/store/hm6iw7j16i1r7ppvfagsyzzrb9zjbq7x-python2.7-setuptools-41.0.1/lib/python2.7/site-packages/site.py", line 26, in __boot
    import imp  # Avoid import loop in Python 3
  File "/nix/store/s5f3vpmig33nk4zyk228q55wdydd3pc2-python3-3.7.3/lib/python3.7/imp.py", line 27, in <module>
    import tokenize
  File "/nix/store/s5f3vpmig33nk4zyk228q55wdydd3pc2-python3-3.7.3/lib/python3.7/tokenize.py", line 33, in <module>
    import re
  File "/nix/store/s5f3vpmig33nk4zyk228q55wdydd3pc2-python3-3.7.3/lib/python3.7/re.py", line 143, in <module>
    class RegexFlag(enum.IntFlag):
AttributeError: module 'enum' has no attribute 'IntFlag'