Set a reading order to the compatibility import file

Set a reading order to the compatibility import file

The order is as follows:
1. Imports, in the same format of "import X"
2. Members denoting versions of python
3. Members of imports that are shadowed for usage across the system
4. Global variables
5. If conditions of imports. (internally organized by this order)
6. Function definitions

Closes: #4409
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4409
Pull-request-sha: a7eb52d39f9758572e6a3314865bb633260aa8e1

Change-Id: I2fa44d1aebd7575df968f2e3f3fc7d474fa3c6d7