Guarantee file closing of input files in optionfiles module

[vtermanis]

With Python3, using the -b parameter (to enable ResourceWarning notifications), one gets the following when loading one or more option files:

mysql-connector/mysql/connector/optionfiles.py:242: ResourceWarning: unclosed file <_io.BufferedReader name='./my.cnf'>

This patch should address the issue by wrapping codes.open in a with statement.

[mysql-oca-bot]

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[vtermanis]

I have submitted a scanned copy of the OCA via email.

[vtermanis]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

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[mysql-oca-bot]

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bug http://bugs.mysql.com/bug.php?id=81519 for updates.
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