Silent failures with psycopg2

[mythmon]

Running pypi2nix -V "3.5" -e 'psycopg2==2.6.1' runs to completion, with no errors, and a 0 exit code, but produces an empty requirements_gerated.txt. The solution is to add -E postgresql, since it is a build time dependency.

[garbas]

tnx for the failing example... that nix-shell stuff does not return correct error code.

[garbas]

but at least now i have a failing example and i can maybe check in the output for this failed things