[deps] Pin PyMySQL dependency

[coveralls]

Pull Request Test Coverage Report for Build 2347

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 82.139%

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Files with Coverage Reduction	New Missed Lines	%
/home/travis/build/chaoss/grimoirelab-elk/grimoire_elk/enriched/github.py	2	75.06%
<!--	Total:	2		-->

Totals
Change from base Build 2343:	-0.02%
Covered Lines:	8333
Relevant Lines:	10145

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💛 - Coveralls