Remove '[testenv] basepython'

It's the cause of many tox 4 bugs and the 'ignore_basepython_conflict'
option is likely to go away at some point [1].

[1] tox-dev/tox#2754 (comment)

Signed-off-by: Stephen Finucane <stephen@that.guru>

tox.ini

@@ -1,10 +1,8 @@
 [tox]
 minversion = 3.1
 envlist = pep8,mypy,clean,py{37,38,39,310,311},report
-ignore_basepython_conflict = true
 
 [testenv]
-basepython = python3
 deps =
   -r{toxinidir}/test-requirements.txt
 commands =