tox: pin basepython to python3.6 in lint-python

A future commit updates the version of `pylint` we use to validate
Python code, which does not work properly with Python 3.9 at the time of
writing, which is the default Python on e.g., a Fedora 32 system.

Instead, pin the Python version to the (Python 3) version as found on
our target system(s), i.e., RHEL/CentOS 7, so Python 3.6.

See: pylint-dev/pylint#3882
See: https://scality.slack.com/archives/CFXQQUDPT/p1608201195013900?thread_ts=1608200122.011200&cid=CFXQQUDPT

tox.ini

@@ -82,6 +82,8 @@ commands =
 [testenv:lint-python]
 description =
     Lint Python files using pylint and mypy.
+# Set Python version to Python3 as shipped with RHEL/CentOS 7
+basepython = python3.6
 deps =
     -r{toxinidir}/buildchain/requirements-{env:OSTYPE}.txt
     -r{toxinidir}/buildchain/requirements-dev.txt