Deprecating Python 3.6 support; one more release...

[peterjc]

• I hereby agree to dual licence this and any previous contributions under both
  the Biopython License Agreement AND the BSD 3-Clause License.

• I have read the CONTRIBUTING.rst file, have run pre-commit locally,
  and understand that AppVeyor and TravisCI will be used to confirm the Biopython unit
  tests and style checks pass with these changes.

• I have added my name to the alphabetical contributors listings in the files
  NEWS.rst and CONTRIB.rst as part of this pull request, am listed
  already, or do not wish to be listed. (This acknowledgement is optional.)

See thread https://mailman.open-bio.org/pipermail/biopython/2020-November/016847.html

[codecov]

Codecov Report

Merging #3426 (c109358) into master (5fd8d0a) will increase coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3426      +/-   ##
==========================================
+ Coverage   84.02%   84.24%   +0.22%     
==========================================
  Files         315      316       +1     
  Lines       51565    52516     +951     
==========================================
+ Hits        43325    44243     +918     
- Misses       8240     8273      +33     

Impacted Files	Coverage Δ
Bio/Phylo/Applications/_Phyml.py	66.66% <0.00%> (ø)
Bio/Phylo/Applications/_Raxml.py	100.00% <0.00%> (ø)
Bio/Phylo/Applications/__init__.py	100.00% <0.00%> (ø)
Bio/SeqIO/TwoBitIO.py	90.51% <0.00%> (ø)
Bio/Seq.py	95.71% <0.00%> (+0.75%)	⬆️
Bio/Phylo/PhyloXMLIO.py	92.22% <0.00%> (+2.25%)	⬆️
Bio/Phylo/NeXMLIO.py	93.16% <0.00%> (+3.40%)	⬆️
Bio/SeqIO/__init__.py	89.95% <0.00%> (+5.38%)	⬆️
Bio/Phylo/Applications/_Fasttree.py	50.00% <0.00%> (+7.14%)	⬆️

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