Fix siunitx format of integer powers with non_int_type=decimal.Decimal

[tovrstra]

• Closes Unexpected output for siunitx format when using Decimal nonint_type #1772
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• (not applicable) Documented in docs/ as appropriate
• Added an entry to the CHANGES file

I've also added venv and .envrc to the .gitignore file:

• venv is created when following the Contributing guide: https://pint.readthedocs.io/en/stable/dev/contributing.html
• .envrc is a file for direnv, which facilitates loading and unloading virtual environments (or any other env variables)

[tovrstra]

(I believe the failed tests are not related to my changes.)

[andrewgsavage]

nice thanks