docs/conf.py: Raise ConfigError for bad Sphinx Python version

Raise ConfigError rather than VersionRequirementError when we detect
that the Python being used by Sphinx is too old.

Currently the way we flag the Python version problem up to the user
causes Sphinx to print an unnecessary Python stack trace as well as
the information about the problem; in most versions of Sphinx this is
unavoidable.

The upstream Sphinx developers kindly added a feature to allow
conf.py to report errors to the user without the backtrace:
  https://github.com/sphinx-doc/sphinx/commit/be608ca2313fc08eb842f3dc19d0f5d2d8227d08


but the exception type they chose for this was ConfigError.

Switch to ConfigError, which won't make any difference with currently
deployed Sphinx versions, but will be prettier one day when the user
is using a Sphinx version with the new feature.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20200313163616.30674-1-peter.maydell@linaro.org