RF: Improve fallback version check, require PyPA packaging module

[effigies]

This moves us fully to PEP440-compliance, and uses the packaging.version module, a carve-out of pip. Using their Version object allows us to avoid manual munging of version strings.

This is a new dependency, but a fairly lightweight one. It reintroduces six and adds pyparsing to our dependency tree. I expect six will probably disappear in a version or two, and pyparsing itself has no dependencies.

Due to the new dependency, this is proposed for 3.1.

Fixes #855.

[codecov]

Codecov Report

Merging #856 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
- Coverage   89.67%   89.66%   -0.02%     
==========================================
  Files          96       96              
  Lines       12398    12382      -16     
  Branches     2187     2184       -3     
==========================================
- Hits        11118    11102      -16     
  Misses        935      935              
  Partials      345      345

Impacted Files	Coverage Δ
nibabel/pkg_info.py	73.91% <100%> (-10.71%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 34384cb...5389364. Read the comment docs.

[effigies]

Anybody up for a review? Principled objections to the packaging dependency?