Fixed pip requirements

[pffs]

PR for fixing #28

(credit to https://stackoverflow.com/a/49867265)

[kadecole]

Any chance this can be merged? This allows you to install when pip version greater than 9 is used. I ran into this issue. The error you will get is below. This PR will resolve this issue.

Collecting pyFG
  Using cached https://files.pythonhosted.org/packages/6f/4d/49cf1489c1bbe654c2589043625b7d103b88e54a2b73d075b697cd3d10aa/pyfg-0.50.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-gxuqzuy9/pyFG/setup.py", line 4, in <module>
        from pip.req import parse_requirements
    ModuleNotFoundError: No module named 'pip.req'
    
    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-gxuqzuy9/pyFG/

[bwmetz]

Was going to submit this myself, but noticed someone else did already.