Fix invalid combination of sizer flags passed to wxPython.
Fix invalid combination of sizer flags passed to wxPython.

In version 4.1, wxPython became more strict about accepting
nonsensical combinations of sizer flags. From the release notes of
wxPython 4.1.1:

   This should have been mentioned in the notes for the last release,
   but alas, it wandered away and got lost. wxWidgets is now
   validating the flags passed when adding items to a sizer, to ensure
   that they are the correct flags for the type of the sizer. If the
   given flags do not make sense, for example using horizontal
   alignment flags in a horizontal box sizer, then a wxAssertionError
   error is raised.

Fix this by removing bogus centering flags in some calls.

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