Better install_requires parsing for skeleton pypi command.

`conda skeleton pypi` didn't work for my packages. It couldn't parse their requirements. The parsing in `pypi.main()` currently only works if `install_requires` is a sequence of single requirement strings. But setup also allows multiple newline separated requirements in one string. And also comments after `#`. I usually just pass the contents of my _requirements.txt_ files to `install_requires`. I extended the code to make that work.