docs/Makefile: work around python-poetry issue #8761

Python-poetry is affected by bug
<python-poetry/poetry#8761>. Namely, if you have
"keyring" <https://pypi.org/project/keyring/> installed, poetry will try
to gain access to the Default collection in the (ex. GNOME) keyring, even
if poetry only needs read-only access to package repositories, and even if
those repos are public.

Consequently, you either unlock your Default collection for poetry
(unjustifiedly), or your GUI session gets effectively locked up, because
any time you hit Cancel on the keyring unlock dialog, poetry immediately
pops up another, and this dialog grabs the keyboard -- you cannot even
switch to a character VT, for killing poetry; you have to log in via ssh
for that.

This issue is not visible to users who don't use "keyring" (GNOME or
otherwise). For those who do, work around the problem by selecting the
"null" keyring back-end, in the environment of every poetry invocation.

Note: I have not regression-tested the workaround in a desktop environment
where "keyring" is unavailable to begin with.

Signed-off-by: Laszlo Ersek <laszlo.ersek@scylladb.com>

docs/Makefile

@@ -1,6 +1,6 @@
 # Global variables
 # You can set these variables from the command line.
-POETRY        = poetry
+POETRY        = PYTHON_KEYRING_BACKEND=keyring.backends.null.Keyring poetry
 SPHINXOPTS    = -j auto
 SPHINXBUILD   = $(POETRY) run sphinx-build
 PAPER         =