so we do not build each swh-* twice with unavaoidable sphinx wrnings when
building each repo locally.

Rather than one big page for each top-level module, containing the apidoc of
all submodules.

This commit slighly modifies Makefiles in order to be able to build
the documentation on Debian stretch (using sphinx 1.4.9) but also
with the latest sphinx version to date (1.7.5).

Changes notably includes:

  - remove the --implicit-namespaces option for sphinx-apidoc as:

     * it is not needed
     * it is not supported on sphinx 1.4.9
     * output is not correct with sphinx 1.7.5

  - use sphinx -b option instead of -M for building html doc
    as an exception is raised otherwise when using sphinx 1.4.9

This is needed because the multiplexer makefile that build apidoc in all
modules explicitly call "make apidoc". Also, it is reasonable to do nothing
anyhow in that case if SKIP_APIDOC=yes is explicitly set.

Makefile.sphinx: also exclude root tests module for apidoc as it may have content defined in its __init__.py (like in swh-web for instance)

This allows you to still have python-sphinx and python3-sphinx coinstalled and
still build with Python 3.

use case: swh-storage