Add __pow__ and __rpow__ to operators.py

[Rolfdv]

By adding pow and rpow to sqlalchemy.sql.operators.py, the Python ** operator can be used on columns, similar to how other native python operators can, such as +, *, /. The addition of __pow__ and __rpow__ follow the same logic as the other operators.

Description

Fixes #8579

Checklist

This pull request is:

• A documentation / typographical error fix
  • Good to go, no issue or tests are needed
• 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!