README.md: Set minimum Python >= 3.8

[cclauss]

symengine.py/setup.py

Line 8 in 810ef47

if sys.version_info[:2] < (3, 8):

[Kibalchish47]

Is this change necessary?

[cclauss]

Is this change necessary?

Yes.

• Require python>=3.8 and bump version to 0.11 #452
• https://devguide.python.org/versions

Supporting five versions of CPython and two versions of PyPy is probably enough complexity.

[certik]

Thanks!