Remove duplicate Python version declarations

ruff can automatically detect this [1] now that it is defined in
pyproject.toml. mypy defaults to the version of Python used to run mypy
[2][3] so we need to keep its configuration around a while longer.

[1] https://docs.astral.sh/ruff/settings/#target-version
[2] https://mypy.readthedocs.io/en/stable/config_file.html#confval-python_version
[3] https://github.com/python/mypy/issues/19349

Change-Id: Ic93d619e5627020fa8665a809b75ea492c1d241b
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

pyproject.toml

@@ -37,7 +37,6 @@ packages = [
 ]
 
 [tool.mypy]
-python_version = "3.10"
 show_column_numbers = true
 show_error_context = true
 ignore_missing_imports = true