Respect python version

[yasirroni]

Current default behavior of pur is to upgrade to latest version available on pypi. It does not respect in what version of Python pur currently used. This cause this kind of error when used in Python3.7,

ERROR: Could not find a version that satisfies the requirement importlib-metadata<7.0.0,>=6.8.0; python_version < "3.8" (from nbmake) (from versions: 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 1.0.0, 1.1.0, 1.1.1, 1.1.2, 1.1.3, 1.2.0, 1.3.0, 1.4.0, 1.5.0, 1.5.1, 1.5.2, 1.6.0, 1.6.1, 1.7.0, 2.0.0, 2.1.0, 2.1.1, 2.1.2, 2.1.3, 3.0.0, 3.1.0, 3.1.1, 3.2.0, 3.3.0, 3.4.0, 3.5.0, 3.6.0, 3.7.0, 3.7.1, 3.7.2, 3.7.3, 3.8.0, 3.8.1, 3.8.2, 3.9.0, 3.9.1, 3.10.0, 3.10.1, 4.0.0, 4.0.1, 4.1.0, 4.2.0, 4.3.0, 4.3.1, 4.4.0, 4.5.0, 4.6.0, 4.6.1, 4.6.2, 4.6.3, 4.6.4, 4.7.1, 4.8.1, 4.8.2, 4.8.3, 4.9.0, 4.10.0, 4.10.1, 4.11.0, 4.11.1, 4.11.2, 4.11.3, 4.11.4, 4.12.0, 4.13.0, 5.0.0, 5.1.0, 5.2.0, 6.0.0, 6.0.1, 6.1.0, 6.2.0, 6.2.1, 6.3.0, 6.4.0, 6.4.1, 6.5.0, 6.5.1, 6.6.0, 6.7.0)
ERROR: No matching distribution found for importlib-metadata<7.0.0,>=6.8.0; python_version < "3.8"

[alanhamlett]

What's your requirements.txt file look like? Do you get this error when running pur or when running pip after running pur?

[alanhamlett]

Related, I verified we're running this code to check an installation candidate's required python version against the current runtime version.