dt-bindings: writing-schema: Install from PyPI repository

Installing dtschema from github.com/devicetree-org is not needed anymore
because dtschema is now part of regular PyPI repository.  In certain
cases it might cause some troubles as it brings latest master version,
not the stable release:

    $ pip3 show dtschema
    Version: 2020.8.2.dev4+g341f3e3
    $ make dt_binding_check
    dtschema minimum version is v2020.8.1

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220223073547.8746-1-krzysztof.kozlowski@canonical.com