Add mysql support

[peter-wangxu]

Keeps same behavior comparing with sqlite3 based SQLiteQueue Fixes #173

[KillahB33]

Hey Peter, here is what my update looks like in the SQLBase.

    def _count(self):
        sql = 'SELECT COUNT({}) FROM {}'.format(
            self._key_column, self._table_name
        )
        row = self._getter.execute(sql).fetchone()
        commit_ignore_error(self._getter)
        return row[0] if row else 0

I have tested it, and it's working. Though I don't know if it will break the other queues

[imidoriya]

I'll try to review this week. Sorry for the delay.

[codecov[bot]]

Codecov Report

Merging #174 (ed37d0a) into master (d6ed33d) will decrease coverage by 0.15%. The diff coverage is 92.24%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
- Coverage   94.05%   93.90%   -0.16%     
==========================================
  Files          15       17       +2     
  Lines        1784     2133     +349     
  Branches      195      225      +30     
==========================================
+ Hits         1678     2003     +325     
- Misses         66       83      +17     
- Partials       40       47       +7     

Flag	Coverage Δ
python	93.90% <92.24%> (-0.16%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
persistqueue/common.py	77.77% <ø> (ø)
persistqueue/exceptions.py	75.00% <ø> (ø)
persistqueue/serializers/json.py	100.00% <ø> (ø)
persistqueue/serializers/pickle.py	88.23% <ø> (ø)
persistqueue/serializers/msgpack.py	73.91% <50.00%> (-6.09%)	:arrow_down:
persistqueue/mysqlqueue.py	84.37% <84.37%> (ø)
persistqueue/sqlbase.py	90.72% <88.19%> (-4.59%)	:arrow_down:
persistqueue/tests/test_mysqlqueue.py	99.01% <99.01%> (ø)
persistqueue/__init__.py	73.33% <100.00%> (+1.90%)	:arrow_up:
persistqueue/pdict.py	98.03% <100.00%> (+0.03%)	:arrow_up:
... and 2 more

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