#6029 Force Six version to 1.12.0

[uilianries]

This is another alternative for #6029. If you don't want to wait for Astroid release, we can force Six version.

Changelog: Bugfix: Fix Six package version to be compatible with Astroid
Docs: Omit
fixes #6029

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[lasote]

This affects also to, at least, 1.17, 1.18, 1.19... So not sure if releasing a minor of 1.20 is going to fix something.

[bldrvnlw]

Is there a recommended workaround while we are waiting for the 1.20.2? (I have an appveyor windows build failing due to this.)
@lasote @uilianries Thanks for all your work on this great product!

[jgsogo]

@bldrvnlw the better workaround I can think about is installing yourself pip install six==1.12.0 before installing Conan. Conan will find a valid six, won't install it and astroid will be satisfied too.

So, in your appveyor:

...
pip install six==1.12.0
pip install conan
...

[bldrvnlw]

@jgsogo Thanks!