Ensure that only x86 interpreters will be used for testing

As support for x64 C-code is not implemented yet (missing compiler support)
and only inprocess address space is implemented for now, python has
to work in 32bit mode, too.

Attentions:
due to bug tox-dev/tox#2535 this
works not work reliable yet.

tox.ini

@@ -1,7 +1,8 @@
 
 [tox]
-envlist=py3{6,7,8,9,10,11}, docs
+envlist=py3{6,7,8,9,10,11}-x86, docs
 skip_missing_interpreters = true
+minversion = 3.15.0
 
 [testenv]
 description = Run UnitTests