Resolve UTF8 charset case-insensitively

[ek247]

Current behavior:

>>> from pymysql.charset import charset_by_name
>>> print(charset_by_name('UTF8'))
None
>>> print(charset_by_name('utf8'))
Charset(id=45, name='utf8mb4', collation='utf8mb4_general_ci')
>>> print(charset_by_name('latin1'))
Charset(id=8, name='latin1', collation='latin1_swedish_ci')
>>> print(charset_by_name('LATIN1'))
Charset(id=8, name='latin1', collation='latin1_swedish_ci')

where I think it's supposed to be able to case-insensitively resolve any utf8 string into utf8mb4