Unbreak math/py-matplotlib when some options are disabled (original PR).

The previous version tried to ${STRIP} non-existing files.

Some additional fixes:

• Fix WXAGGBACKEND_VARS, which was overwritten and broke the WX build
• Fix permissions in .py files
• Sort entries in post-install
• Whitespace fixes

PR: 203417
Submitted by: Tomi Kause
Reviewed by: olgeni
Approved by: maintainer