Issue 6288: update the contextlib.nested() docs to explain why it has been deprecated and should generally be avoided

Ensure that the snippet in doctest_aliases actually contains aliases.

(On Unix) Patch by STINNER Victor.

The rational is the change might cause imcompatiblity problems with
PyYAML. In addition, Raymond wants to kept the different versions of
collections synchronized across Python versions.

PyErr_SetFromErrnoWithUnicodeFilename.

(with the right revisions this time, hopefully)

Fixes #6258.