Remove support for Python <=3.7

[MilesCranmer]

According to https://endoflife.date/python, Python 3.7 has been unsupported since June 27, 2023 which includes any security patches.

Therefore I suggest that PyCall.jl drop support for Python <=3.7 as these versions of Python should not be used anyways due to the security risks. (If a user really needs to then they can always install an older version of PyCall.jl)

x-ref discussion with @stevengj in #1068