Revert "tox: Remove basepython and ineffective ignore_basepython_conflict"

These are needed as we need to override PYTHON3_VERSION
in FIPS job running with CentOS 9-stream.

This reverts commit b615560926.

Signed-off-by: Yatin Karel <ykarel@redhat.com>
Change-Id: Id49739b36f16a05a0ce8d702ed390c56ce392c46

tox.ini

@@ -1,10 +1,14 @@
 [tox]
 envlist = docs,py3,pep8
 minversion = 4.6.0
+ignore_basepython_conflict = True
 
 [testenv]
 description =
   Run unit tests.
+# NOTE(ykarel): we need this to override python3 version
+# in FIPS job running on CentOS 9-stream
+basepython = {env:TOX_PYTHON:python3}
 setenv =
   VIRTUAL_ENV={envdir}
   OS_LOG_CAPTURE={env:OS_LOG_CAPTURE:true}