[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>

requirements-dev.txt

@@ -20,4 +20,4 @@ sphinx-tabs==1.1.13; python_version < '3.6'
 sphinxcontrib-programoutput==0.16
 sphinx-autobuild==2020.9.1; python_version >= '3.6'
 sphinx-autobuild==0.7.1; python_version < '3.6'
-linuxdoc @ git+http://github.com/return42/linuxdoc.git@70673dcf69e705e08d81f53794895dc15c4920b3#egg=linuxdoc
+linuxdoc==20210110