Added python_version environment marker for install_requires to fix situation when allure-python-commons dependency enum34 was added to universal wheel file without proper handling of python version.

Context

Currently, enum34 dependency for allure-python-commons is added depending on python version of build system instead of python version of setup destination.

This issue can be reproduced by installation of allure-pytest==2.2.3b1 from PyPi whl file on python 3.6. This will install enum34 library for 3.6 system and cause internal python exceptions, like:

  File "/opt/jenkins/shiningpanda/jobs/af2a4440/virtualenvs/d41d8cd9/bin/coverage", line 4, in <module>
    import re
  File "/opt/jenkins/shiningpanda/jobs/af2a4440/virtualenvs/d41d8cd9/lib/python3.6/re.py", line 142, in <module>
    class RegexFlag(enum.IntFlag):
AttributeError: module 'enum' has no attribute 'IntFlag'```