Remove coding: utf-8 per PEP 3120

[levbishop]

utf-8 has been the default since PEP 3120.
https://www.python.org/dev/peps/pep-3120/

Non trivial edits are to Makefile and verify_headers.py

[mtreinish]

I always assumed that line was for emacs users since I thought this was the variable format used by emacs: https://www.gnu.org/savannah-checkouts/gnu/emacs/manual/html_node/emacs/Specifying-File-Variables.html#Specifying-File-Variables these lines never did anything for me in vim :P. That being said this LGTM besides the pep this has come up on windows before for release notes and documentation (mostly around the text drawer) so we're explicitly utf8 for all text files in the repo (it might be worth calling that out in the CONTRIBUTING.md)

[jlapeyre]

Apparently (https://docs.python.org/3/howto/unicode.html) the syntax for specifying encoding in python was borrowed from emacs. But, as noted in the link, if the encoding is omitted, then utf-8 is assumed.