Dynamically computed install_requires: investigate potential problems

[kuba]

In #1661 (comment) @mgedmin said:

Hmm. Dynamically-computed install_requires causes problems for users of pip >= 7.0, which has wheel caching enabled by default: pypa/pip#3025. Short version:

• I pip install letsencrypt on Python 2.7, pip runs setup.py and builds a wheel (marked -py2-none-any.whl) and puts it in ~/.cache/pip/
• I pip install letsencrypt on Python 2.6, pip discovers a valid wheel and installs it, using install_requires computed earlier for a different Python version."
  Conditional python-ndg-httpsclient dependency doesn't work on Ubuntu 14.04 #2175 might be related

[Natim]

In #2175 I added an extra_requires rule that will be taken into account when installing wheel and make it possible to build universal wheel.