Drop support for Python 3.6, specify python_requires

[pllim]

https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/#python-requires

• Python 3.6 has reached EOL, so there is no reason to support it.
• Please specific python_requires. Recommended now is 3.7, or even 3.8 if you do not really need to run anything on 3.7.

https://github.com/astropy/astropy.github.com/pull/491#issuecomment-1215349065

[Jammy2211]

Done https://github.com/Jammy2211/PyAutoGalaxy/blob/main/setup.py