Hide just default alphabet in Seq __repr__

[peterjc]

This pull request addresses issue #1674, taking part of the work on #1676, which expanded in scope to remove the alphabet from the Seq-objects' __repr__ completely.

This only hides the alphabet if it is the default alphabet. I would like to include this in Biopython 1.72

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 and understand that AppVeyor and TravisCI will be used to confirm the Biopython unit tests and flake8 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.)

[codecov]

Codecov Report

Merging #1687 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1687      +/-   ##
==========================================
- Coverage   85.52%   85.46%   -0.06%     
==========================================
  Files         327      327              
  Lines       50039    50082      +43     
==========================================
+ Hits        42795    42805      +10     
- Misses       7244     7277      +33

Impacted Files	Coverage Δ
Bio/Seq.py	98.12% <100%> (+0.02%)	⬆️
Bio/Blast/NCBIWWW.py	9.24% <0%> (-2.53%)	⬇️

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