Remove support for python2

[peter-wangxu]

1.0.0-alpha

1. Only Python3.x series are offically supported by persistqueue, since Python 2 was no longer under maintenance since 2020

2. persistqueue.Queue using serializer=persistqueue.serializers.pickle created under python2 was no longer able to be read by persist queue after 1.0.0
   as the default pickle version changed from 2 to 4

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (74f464a) 93.65% compared to head (d58f253) 95.04%.

Files	Patch %	Lines
persistqueue/serializers/msgpack.py	77.77%	0 Missing and 2 partials ⚠️
persistqueue/mysqlqueue.py	94.11%	1 Missing ⚠️
persistqueue/queue.py	96.96%	1 Missing ⚠️
persistqueue/serializers/pickle.py	91.66%	0 Missing and 1 partial ⚠️
persistqueue/sqlackqueue.py	96.96%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
+ Coverage   93.65%   95.04%   +1.38%     
==========================================
  Files          18       17       -1     
  Lines        2208     2178      -30     
  Branches      230      224       -6     
==========================================
+ Hits         2068     2070       +2     
+ Misses         89       60      -29     
+ Partials       51       48       -3     

Flag	Coverage Δ
python	95.04% <96.79%> (+1.38%)	⬆️

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