Improve write_pyi usage

[agriffin-grow]

Fixes #1524

Description

Due to some problems when running write_pyi, this includes the following changes:

1. move the sys.path.append up before alembic is imported - this is using global imports otherwise
2. use tox to run write_pyi in order to avoid globally installed dependencies

Checklist

This pull request is:

• [ ] A documentation / typographical error fix
  • Good to go, no issue or tests are needed
• [x] A short code fix
  • please include the issue number, and create an issue if none exists, which must include a complete example of the issue. one line code fixes without an issue and demonstration will not be accepted.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests. one line code fixes without tests will not be accepted.
• [ ] A new feature implementation
  • please include the issue number, and create an issue if none exists, which must include a complete example of how the feature would look.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests.

Have a nice day!

[sqla-tester]

New Gerrit review created for change 679ea5c67ebbf5062891d50dabf6323cf5348d82: https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/5586

[sqla-tester]

Gerrit review https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/5586 has been merged. Congratulations! :)