Adds exception when trying to run write_pyi.py with Python < 3.9

Adds exception when trying to run write_pyi.py with Python < 3.9

### Description
write_pyi.py now raises an exception when someone tries to run the script with Python version 3.8 or smaller.

This also fixes:
- When using absolute Paths, i.e. on Windows, they are now converted to relative paths to avoid useless changes in the doc strings of the pyi files
- Corrected destination_path argument type when calling `generate_pyi_for_proxy` in `run_file`
- A comment typo

Fixes: #915
Closes: #919
Pull-request: https://github.com/sqlalchemy/alembic/pull/919
Pull-request-sha: e885d27ac5509858c8cb2927310f6195fc1d6004

Change-Id: Ia33d32b2828e71185186afc49848ce89d48a8323