pip: Rename flatpak-pip-generator to flatpak-pip-generator.py

[lwbt]

Rename flatpak-pip-generator to flatpak-pip-generator.py to be able to run it with pipx.

$  pipx run https://github.com/flatpak/flatpak-builder-tools/raw/master/pip/flatpak-pip-generator
pipx will only execute apps from the internet directly if they end with '.py'. ...

$  pipx run https://github.com/lwbt/flatpak-builder-tools/raw/master/pip/flatpak-pip-generator.py
Please specifiy either packages or requirements file argument

flatpak-poetry-generator.py

pipx run https://github.com/flatpak/flatpak-builder-tools/raw/master/poetry/flatpak-poetry-generator.py
usage: -c [-h] [-o OUTFILE] [--production] lockfile
-c: error: the following arguments are required: lockfile

---

I also fixed the typo I just found while creating the PR.