Add --setup-requires option to install pip packages before building wheels.

[La0]

Needed to build scipy.
I tested it with this command in nix-shell:

TMPDIR=../tmp pypi2nix -v -V "3.5" -e scipy --pre-install "numpy" -E "openblas"

Pip does not complain anymore about numpy, but fails on a missing library (openblas), even if this lib. is set in the extra build inputs.

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This change is

[garbas]

i lost inline comments i wrote since FF crashed :(

anyway here is the shortlist how i see we need to move here:

• setup requires should be handled within stage0 (pip.nix)
• i already had a plan to separate pip wheel into pip install and pip wheel where pip wheel would only view sources from first pip install. this would also allow us to cache creation of wheels (should speed sequential runs by 50-90%)
• i think option should be called --setup-requires since that is what setuptools calls it (i'm currently working splitting --editable options into --install-requires and --editable since some found it confusing that one option did 2 things
• this new setup_requires option should be also part of buildInputs

[garbas]

i'm working on some "UI" changes which will change how options are called and i'm adding better description to --help with few examples how to use pypi2nix.

[garbas]

i'm merging this, but i need to do some fixes as well in afterward commits. tnx!