I know that having to rely on PYTHONPATH has been a constant source of pain, but what problems does this solve? (I honestly don't know :))

We can export NIX_PYTHONPATH without breaking support for PYTHONPATH.

this seems to cause the package to appear twice during the pythonCatchConflictsPhase

checking for references to /build/ in /nix/store/1ysh1pxvgh71zkzsbglbwcrvamnw88n0-python3.7-atomicwrites-1.3.0...
pythonCatchConflictsPhase
Found duplicated packages in closure for dependency 'atomicwrites':
  atomicwrites 1.3.0 (/nix/store/1ysh1pxvgh71zkzsbglbwcrvamnw88n0-python3.7-atomicwrites-1.3.0/lib/python3.7/site-packages)
  atomicwrites 1.3.0 (/build/atomicwrites-1.3.0)

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/761cgkm5nn73rs6vbw4n2ljh1nmbdi6l-python3.7-atomicwrites-1.3.0.drv' failed with exit code 1
cannot build derivation '/nix/store/5x18wwnk4dnxx3wxw00jmzhr6lhrn1an-python3.7-pytest-5.1.0.drv': 1 dependencies couldn't be built
error: build of '/nix/store/5x18wwnk4dnxx3wxw00jmzhr6lhrn1an-python3.7-pytest-5.1.0.drv' failed

Also, NIX_PYTHONPATH gets unset immediately, so it would not break #71110.

Thank you for your contributions.
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