chore: register custom pytest markers to snuff error messages

[ChristianLieven]

• [x] description of feature/fix
• [x] add an entry to the next release

This removes the warnings that appear with the most recent version of Pytest under Python 3.6.0 by registering the custom markers we use for parametrization (biomass, essentiality and growth).

Running memote report snapshot on Google Colaboratory (Python 3.6.8) errors with something along these lines: 'biomass' not found in `markers` configuration option

This is recommended by the authors of pytest

[Midnighter]

We probably have to drop Python 2 support since the scientific Python dependencies have already done so.

[ChristianLieven]

drop py2 support

The sooner the better! I'm happy to make a quick and dirty switchover ASAP (import warning and removal of py2 CI). We should probably discuss steps on how to do a full transition (similar to solution 3 suggested here: https://github.com/opencobra/cobrapy/issues/853#issuecomment-515514559)

[codecov[bot]]

Codecov Report

Merging #670 into develop will decrease coverage by 0.05%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #670      +/-   ##
===========================================
- Coverage    82.59%   82.54%   -0.06%     
===========================================
  Files           49       49              
  Lines         2597     2601       +4     
  Branches       424      424              
===========================================
+ Hits          2145     2147       +2     
- Misses         366      368       +2     
  Partials        86       86

Impacted Files	Coverage Δ
src/memote/suite/collect.py	83.78% <100%> (+0.92%)	:arrow_up:
src/memote/suite/cli/runner.py	51.81% <0%> (-0.52%)	:arrow_down:

---

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 709563d...86a52db. Read the comment docs.