setup: Check for setuptools version and inform user

[HerrMuellerluedenscheid]

fixes #83

debian jessie's (and maybe others?) apt-get ships a rather deprecated version of setuptools (5.5.1). Setup of pyota failed with a rather unclear exiting message:

X@Y:/usr/local/src/pyota$ sudo python setup.py install 
error in PyOTA setup command: 'install_requires' must be a string or list of strings containing valid 
project/version requirement specifiers

Same with installing via pip. I went through the version log of setuptools and found 'install_requires' was touched in versions 5.5, 20.5. and later. I tested with version 20.7. with which setup was flawless. So, I assume version 20.5. is the minimum requirement.

[todofixthis]

Hey @HerrMuellerluedenscheid thanks for your pull request! This is a great idea!

Couple of requests:

• Could you change your PR so that it uses develop as the target branch?
• Could you modify the error message so that it indicates the minimum setuptools version number?

[todofixthis]

Thanks @HerrMuellerluedenscheid ! Will do a final review later today.

[todofixthis]

Looks good! Thanks!!