Add Path for type of read_default_file connection parametr

Sample code:

from pathlib import Path
import MySQLdb

conn = MySQLdb.connect(
    read_default_file=Path(__file__).with_name('mylogin.cnf').resolve())

Error:

Traceback (most recent call last):
  File "<input>", line 2, in <module>
  File "/home/test/projects/venv/lib/python3.6/site-packages/MySQLdb/__init__.py", line 85, in Connect
    return Connection(*args, **kwargs)
  File "/home/test/projects/venv/lib/python3.6/site-packages/MySQLdb/connections.py", line 208, in __init__
    super(Connection, self).__init__(*args, **kwargs2)
TypeError: connect() argument 12 must be str, not PosixPath

Reproduce the sample using pathlib.Path with MySQLdb.connect, then start at MySQLdb/init.py and connections.py, as shown in the traceback. Check how read_default_file reaches the connection layer and verify that Path input no longer raises the reported TypeError while existing string input still works.