[tests] uninstall Python 3.5 first
[tests] uninstall Python 3.5 first

No glue why 3.5.4 has to be uninstalled first
but it seems to be necessary to install Python 3.5.0

https://ci.appveyor.com/project/Ladsgroup/pywikibot-g4xqx/builds/37689998/job/p2gbfmx3lnvjn39d

Bug: T273367
Change-Id: I82f8c4a2f448f86b48ccd9366b24552ab5e5bee9

wikimedia/pywikibot-core,wikimedia/pywikibot-core