Allow is_flag=True, multiple=True with non-bool flag_value

[hashstat]

Allow creating options with is_flag=True and multiple=True if flag_value is also set to a non-bool value.

Fixes #2292

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [ ] Add or update relevant docs, in the docs folder and in code.
• [x] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [ ] Add .. versionchanged:: entries in any relevant code docs.
• [x] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.