Prepare version 5.0 release

[mmore500]

• drop Python2.7 support
• drop Python3.5 support
• NOT PLANNED drop Python3.6 support
• NOT PLANNED finalize pending deprecations
• smoke tests and any critical unit tests
• fix layout of badges on README

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0.35800% with 835 lines in your changes are missing coverage. Please review.

Project coverage is 88.73%. Comparing base (2eb8c26) to head (5c84b44).

Files	Patch %	Lines
tests/smoketests/test_container.py	0.00%	110 Missing ⚠️
tests/smoketests/test_coalescent.py	0.00%	72 Missing ⚠️
tests/smoketests/test_treesum.py	0.00%	69 Missing ⚠️
tests/smoketests/test_messaging.py	0.00%	52 Missing ⚠️
tests/smoketests/test_taxonmodel.py	0.00%	47 Missing ⚠️
tests/smoketests/test_charmatrixmodel.py	0.00%	46 Missing ⚠️
tests/smoketests/test_treecollectionmodel.py	0.00%	40 Missing ⚠️
tests/smoketests/test_reconcile.py	0.00%	36 Missing ⚠️
tests/smoketests/test_probability.py	0.00%	32 Missing ⚠️
tests/smoketests/test_filesys.py	0.00%	31 Missing ⚠️
... and 25 more

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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #176       +/-   ##
===========================================
+ Coverage   58.79%   88.73%   +29.93%     
===========================================
  Files         108      114        +6     
  Lines       19851    12877     -6974     
===========================================
- Hits        11672    11426      -246     
+ Misses       8179     1451     -6728     

Flag	Coverage Δ
unittests	88.73% <0.35%> (+29.93%)	⬆️

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