Allow git builds of Sphinx

Allow git builds of Sphinx

When using a non-release version of Sphinx, from a local build (with
improvements for kernel doc handling, why not),

	sphinx-build --version

reports versions of the form

	sphinx-build 3.0.0+/2acc79f58ff2

i.e. base version, a plus symbol, slash, and the start of the git hash
of whatever repository the command is run in (no, not the hash that
was used to build Sphinx!).

This patch fixes the installation check in sphinx-pre-install to
recognise such version output.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20200124183316.1719218-1-steve@sk2.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>