Use packaging.version instead of pkg_resources.parse_version by default

[felixonmars]

packaging.version is the new standard and should be preferred now. It
reduces startup time significantly too as pkg_resources is expensive to
import.

parse_version is only kept for compatibility here, perhaps consider
removing it in the future.

[felixonmars]

The test failures are irrelevant.

[lilydjwg]

The issue with packaging.version is that, it's designed to work with Python packages' versions only. Something like R2020 or 1.1.1.k will fall back to LegacyVersion which is going to be removed soon.

We may need to implement a loose version comparison algorithm ourselves.

[yan12125]

Test failures are addressed in #181

[felixonmars]

Something like R2020 or 1.1.1.k will fall back to LegacyVersion which is going to be removed soon.

pkg_resources.parse_version uses the same code here, IIRC. (The LegacyVersion warning is also present in current test implementation.)