Support Python 3.9, drop Python 3.6

[MarcoGorelli]

No description provided.

[codecov]

Codecov Report

Merging #4332 (66d1608) into master (f4552e3) will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4332      +/-   ##
==========================================
+ Coverage   87.67%   87.87%   +0.19%     
==========================================
  Files          88       88              
  Lines       14240    14473     +233     
==========================================
+ Hits        12485    12718     +233     
  Misses       1755     1755              

Impacted Files	Coverage Δ
pymc3/backends/tracetab.py	100.00% <ø> (ø)
pymc3/data.py	81.01% <ø> (+0.08%)	⬆️
pymc3/model.py	89.28% <ø> (+0.38%)	⬆️
pymc3/distributions/bart.py	80.15% <0.00%> (-0.65%)	⬇️
pymc3/parallel_sampling.py	86.75% <0.00%> (-0.05%)	⬇️
pymc3/sampling.py	89.69% <0.00%> (ø)
pymc3/sampling_jax.py	0.00% <0.00%> (ø)
pymc3/tests/conftest.py	100.00% <0.00%> (ø)
pymc3/variational/stein.py	100.00% <0.00%> (ø)
pymc3/distributions/continuous.py	93.31% <0.00%> (+<0.01%)	⬆️
... and 34 more

[twiecki]

LGTM, just needs a line in the release-notes.

[MarcoGorelli]

Suggestions on how to phrase it?

## PyMC3 4.0.0 (on deck)

This is the first release to support Python3.9 and to drop Python3.6.

### Maintenance
- Fixed bug whereby partial traces returns after keyboard interrupt during parallel sampling had fewer draws than would've been available [#4318](https://github.com/pymc-devs/pymc3/pull/4318)
- Make `sample_shape` same across all contexts in `draw_values` (see [#4305](https://github.com/pymc-devs/pymc3/pull/4305)).
- Removed `theanof.set_theano_config` because it illegally touched Theano's privates (see [#4329](https://github.com/pymc-devs/pymc3/pull/4329)).

[twiecki]

I think we can put this into 3.11. CC @michaelosthege

[michaelosthege]

I will change this in #4214 to become 3.11.