fix: support Python 3.7 again

[grische]

I have tested Python 3.7 with the python:3.7 Docker container using the following commands

python3.7 -m venv .venv
. .venv/bin/activate
python3.7 -m pip install .
python3.7 -m pip install .[test]
python3.7 -m pytest .
python3.7 -m pip install .[lint]
python3.7 -m ruff check

and then used the following ini and put it into ./package-index/morgan.ini

[env.windows-py39]
os_name = nt
platform_tag = win_amd64
sys_platform = win32
platform_machine = AMD64
platform_python_implementation = CPython
platform_system = Windows
python_version = 3.9
python_full_version = 3.9.7
implementation_name = cpython

[requirements]
extremeflash = >0.5

and ran the mirroring to check both if packaging 24.0 worked and morgan is properly installed as well as the actual mirror:

morgan mirror --index-path ./package-index

This fixes #40
This closes #55