[Chapel Merge] Update docs prereqs required Python version to 3.7

Branch: refs/heads/main
Revision: 51c355f
Author: bmcdonald3
Log Message:

Merge pull request #18374 from bmcdonald3/prereqs

Update docs prereqs required Python version to 3.7 for the Chapel test environment

[trivial, not reviewed]

A new dependency for c2chapel, pycparserext, requires Python 3.7, so this doc change now properly reflects that requirement.

Modified Files:
M doc/rst/usingchapel/prereqs.rst

Compare: https://github.com/chapel-lang/chapel/compare/d67222e0cfc9...51c355ff5445