Prepare version 5.0 release

[mmore500]

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

[mmore500]

python_requires='>=3.6',

[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 :warning:
tests/smoketests/test_coalescent.py	0.00%	72 Missing :warning:
tests/smoketests/test_treesum.py	0.00%	69 Missing :warning:
tests/smoketests/test_messaging.py	0.00%	52 Missing :warning:
tests/smoketests/test_taxonmodel.py	0.00%	47 Missing :warning:
tests/smoketests/test_charmatrixmodel.py	0.00%	46 Missing :warning:
tests/smoketests/test_treecollectionmodel.py	0.00%	40 Missing :warning:
tests/smoketests/test_reconcile.py	0.00%	36 Missing :warning:
tests/smoketests/test_probability.py	0.00%	32 Missing :warning:
tests/smoketests/test_filesys.py	0.00%	31 Missing :warning:
... and 25 more

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Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/jeetsukumaran/DendroPy/pull/176/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeet+Sukumaran) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/jeetsukumaran/DendroPy/pull/176/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeet+Sukumaran) | `88.73% <0.35%> (+29.93%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeet+Sukumaran#carryforward-flags-in-the-pull-request-comment) to find out more.

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