Drop support for Python 3.6 and Python 3.7

[sondreso]

Issue
Python 3.6 is EOL and Python 3.7 support has already been dropped by popular packages such as numpy. We also no longer need to support Python 3.6/3.7 for internal deploys.

Approach
Drop support for Python 3.6 and 3.7

Note:
Required tests will be changed after this is approved

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[codecov-commenter]

Codecov Report

Merging #3490 (db75e81) into main (a1eab17) will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##             main    #3490   +/-   ##
=======================================
  Coverage   64.97%   64.97%           
=======================================
  Files         601      601           
  Lines       47399    47391    -8     
  Branches     4114     4114           
=======================================
- Hits        30796    30791    -5     
+ Misses      15299    15293    -6     
- Partials     1304     1307    +3     

Impacted Files	Coverage Δ
ert_shared/models/types.py	0.00% <0.00%> (ø)
..._shared/ensemble_evaluator/narratives/narrative.py	84.63% <100.00%> (+0.30%)	⬆️
libres/lib/res_util/block_fs.cpp	67.27% <0.00%> (-0.92%)	⬇️

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[berland]

✂️ 👍🏻

[berland]

There are more things that should be cleaned up in this PR. Do a grep for sys.version and a handful of cases will be found.

[berland]

Fix more sys.version issues.

[sondreso]

Jenkins test this please