Support External Classes' __rtruediv__

[michaeljpeters]

What do these changes do?

Allows external classes to extend functionality properly using __rtruediv__.

Previously, URL would raise a not-so-nice error like this:

>>> from yarl import URL
>>> 
>>> u = URL('https://beefslab.com/abc')
>>> u
URL('https://beefslab.com/abc')
>>> u / 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/peters/.env38/lib/python3.8/site-packages/yarl/_url.py", line 318, in __truediv__
    name = self._PATH_QUOTER(name)
  File "yarl/_quoting_c.pyx", line 213, in yarl._quoting_c._Quoter.__call__
TypeError: Argument should be str

Now, it will properly return NotImplemented (still eventually resulting in a TypeError).

>>> u / 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for /: 'URL' and 'int'

https://docs.python.org/3/library/constants.html#NotImplemented
https://docs.python.org/3/reference/datamodel.html#object.__rtruediv__
"These functions are only called if the left operand does not support the corresponding operation"

Are there changes in behavior for the user?

No

Related issue number

#832

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes - Not added, minor change
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.