Rule request

Inspired by https://www.python.org/dev/peps/pep-0597

The PEP is still draft, but I think WPS can implement before adoption. Even if the PEP is rejected I think this is a good idea.

Required specifying "encoding" to open. Even if it's just None.

Thesis

Not specifying the encoding could be considered a bug. Making the effort to choose which encoding is better. At the moment there is no builtin "locale" so we have to settle for None or call locale.getpreferredencoding() all the time. The second option is a bit clumbersome.

## Wrong
with open('filename.txt') as fd:
    fd.read()

## Correct
with open('filename.txt', encoding='ascii') as fd:
  fd.read()

with open('filename.txt', encoding='utf8') as fd:
  fd.read()

# Unfortunately we have to settle for "None" to indicate "locale".
with open('filename.txt', encoding=None) as fd:
  fd.read()

Reasoning

Everything described in the PEP.