Sort input file list

[bmwiedemann]

Sort input file list
so that _libiminuit.so builds in a reproducible way
in spite of indeterministic filesystem readdir order
and http://bugs.python.org/issue30461

See https://reproducible-builds.org/ for why this is good.

This PR was done while working on reproducible builds for openSUSE.

[codecov]

Codecov Report

Merging #355 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #355   +/-   ##
========================================
  Coverage    65.71%   65.71%           
========================================
  Files          137      137           
  Lines         4431     4431           
  Branches       651      651           
========================================
  Hits          2912     2912           
  Misses        1281     1281           
  Partials       238      238

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[HDembinski]

If I understand correctly, this change may help to make all binaries from the same build environment identical, which is certainly a good thing. Thank you for the PR!

[bmwiedemann]

Yes, that is the goal.
Thanks for merging.