[Chapel Merge] Fix problem with sphinx-building having too-long s

Branch: refs/heads/master
Revision: 22e5eda
Author: mppf
Log Message:

Merge pull request #16657 from mppf/fix-too-long-sphinxbuild

Fix problem with sphinx-building having too-long shebang

We were seeing test failures with make docs due to the sphinx-build
in the venv having a shebang that is too long.

So, run it with python3 path/to/sphinx-build. A reasonable alternative
would be to run third-party/chpl-venv/venv-use-sys-python.py all the
time instead of in certain configurations.

While there, this PR improves the checking for a sphinx-build to check
that sphinx-build --version works.

Modified Files:
M doc/Makefile.sphinx

Compare: https://github.com/chapel-lang/chapel/compare/30809a2fa1bd...22e5eda265ac