Also fixes retworkx Darwin build.

Fixes linker error for retworkx that required libiconv.
Error log:
error: linking with `/nix/store/47vpv5i10dwfg1cf5wca1k40f982g5fm-clang-wrapper-7.1.0/bin/cc` failed: exit code: 1
    ...
    ld: library not found for -liconv
    clang-7: error: linker command failed with exit code 1 (use -v to see invocation)

Also fix tests running on build, that didn't get converted when switched
to the new way of running Cargo builds for python packages.

Also unbreak build on master by disabling tests that compare visual
matplotlib images.

Also remove force deletes from postCheck.

Use a patch submitted via PR instead of custom substitute.

Remove pybind11 install requirement, as requested by @dotlambda

Also remove unused patch & unneeded docs delete (they're not in $out).

Remove unneeded docs delete, they're no longer in $out.

Add optional subpackages as arguments. Most of these are not in nixpkgs,
but can be found in https://github.com/drewrisinger/nur-packages/.