Spglib: fix warnings

spglib changes so as to use proper exceptions whereas previously it would return None on failure.

That's very good but we now get a lot of warnings whenever we call spglib saying that we need to opt either in or out of this new behaviour.

Let's opt in then. This MR adds a wrapper which, for new spglib, silences the warning by setting an envvar. If spglib does return None (indicating old spglib) then we promote it to an exception.

Closes: #1858 (closed)

(The mechanism here is not thread-safe but threading does not seem to be a common use case, at least we haven't done much for thread safety.)

Checklist: x (done) or ~ (irrelevant)

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Changes have been added in changelog.d using scriv.
• Issue is resolved via "closes #XXXX" if applicable.