Deprecate Python 3.6 support, v0.14 last supported version

[CasperWA]

Since it's not critical to drop Python 3.6 anymore, it's nicer with a grace period. Even though it may be short.

This PR implements a DeprecationWarning explaining that v0.14 will be the last version to support Python 3.6, and that one should update to Python 3.7+ to receive v0.15 and onwards.

[codecov]

Codecov Report

Merging #751 (9048578) into master (46c9a8f) will decrease coverage by 0.07%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #751      +/-   ##
==========================================
- Coverage   92.79%   92.72%   -0.08%     
==========================================
  Files          68       68              
  Lines        3663     3668       +5     
==========================================
+ Hits         3399     3401       +2     
- Misses        264      267       +3     

Flag	Coverage Δ
project	92.72% <40.00%> (-0.08%)	⬇️
validator	92.72% <40.00%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optimade/__init__.py	57.14% <40.00%> (-42.86%)	⬇️

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[ml-evs]

Feel free to merge after fixing

[CasperWA]

Cheers guys. I'm trying to force it to show. Maybe I should just make it a print statement instead?

[shyamd]

Yeah, a print will show no matter what, vs this which can be and by default is filtered I believe.

[CasperWA]

Yeah, a print will show no matter what, vs this which can be and by default is filtered I believe.

Indeed. I tried wrapping it in a context where I changed the warnings filter, but I believe it also overwrites the showarning() function, so it essentially doesn't matter. I'll change to a print statement instead, as I think it's important this is shown upon install.

[CasperWA]

Through some testing, I've found that it's extremely difficult to get output in stdout/stderr. I think instead I'll put a warning when the module is imported. It should only show once.

[CasperWA]

The message now shows up once when anything is imported from the optimade package. It can, in theory, be ignored by prepending another warning filter (or removing the one I put in), but I think that's fine?