Allow specifiying a timezone for connections

[Nothing4You]

Hi there,
please implement setting the timezone on a connection level, similar to the way mysql.connector works with time_zone.

This allows ensuring a constant timezone like UTC independent from what the database might have configured.

Related: aio-libs/aiomysql#235

[methane]

FYI, init_command="SET SESSION time_zone=UTC" works.

[guyskk]

FYI, init_command="SET SESSION time_zone=UTC" works.

Use init_command="SET SESSION time_zone='+00:00'"
to avoid pymysql.err.InternalError: (1298, "Unknown or incorrect time zone: 'UTC'")