Add shebangs to pyx/pxd/pyx.in files
This enables automatic syntax highlighting in SublimeText (and presumably other editors).  This is effectively 100% cosmetic.

`msgpack` had shebangs `# coding: utf-8` which is changed to `# -*- coding: utf-8 -*-`.  If there is a reason to use the former, let me know and I'll amend the PR. 

Related: #15131, #15134

 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [X] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
 - [ ] whatsnew entry
