[fix] requirements-dev.txt use linuxdoc==20210110 from pypi

Fore good reasons (force push) PyPi does not accept a git commit as a
requirement [1].  Today linuxdoc has been released on pypi, lets use it.
With this patch issue [#2393] should be obsolete.

[#2393] https://github.com/searx/searx/issues/2393



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>