making setup.py PEP-8 compliant
Renaming encoding `UTF-8` to `utf-8` in all the test files.

UTF-8 is not recognized by emacs and therefore adds an unpleasant
friction to write tests in it.

This is the emacs error message when using encoding UTF-8:

Warning (mule): Invalid coding system `UTF-8' is specified
for the current buffer/file by the :coding tag.
It is highly recommended to fix it before writing to a file.