Describe the bug
The build in pypi lack the constants directory, causing the module to be unusable

To Reproduce
Complete steps to reproduce the behavior:
install pymysql 1.0.3rc1

Code:

>>> import pymysql
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pymysql/__init__.py", line 26, in <module>
    from .constants import FIELD_TYPE
ModuleNotFoundError: No module named 'pymysql.constants'

Expected behavior
import succeed

Environment

• OS: linux
• PyMySQL version: 1.0.3rc1

Additional context
N/A