[SQLAlchemy] support SHOW collation

[bzz]

Following discussion from #319 (comment)

SQLAlchemy uses SHOW collation statement:

• in _detect_collations
• in dialects/mysql/mysqldb.py
  show collation where Charset = 'utf8mb4' and Collation = 'utf8mb4_bin'

Here is a python snippet to reproduce this.