[3.6] bpo-32568: make select.epoll() and its docs consistent (GH-7840) (GH-8025)

* `flags` is indeed deprecated, but there is a validation on its value for
  backwards compatibility reasons.  This adds mention of this in the docs.
* The docs say that `sizehint` is deprecated and ignored, but it is still
  used when `epoll_create1()` is unavailable. This adds mention of this in
  the docs.
* `sizehint=-1` is acceptable again, and is replaced with `FD_SETSIZE-1`.
  This is needed to have a default value available at the Python level,
  since `FD_SETSIZE` is not exposed to Python. (see: bpo-31938)
* Reject `sizehint=0` since it is invalid to pass on to `epoll_create()`.

The relevant tests have also been updated.

(cherry picked from commit 0cdf5f42898350261c5ff65d96334e736130780f)

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