docs: force sphinx version above 3.0 to avoid caching in RTD

ReadTheDocs somehow caches requirements and insists in using old
version of Sphinx (1.8). We have to force using newer one (4.5)

Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>

doc/requirements.txt

@@ -1,6 +1,6 @@
 wheel
 
-sphinx
+sphinx > 3.0
 
 # convert markdown to rest
 # m2r2 fork is active in development and works with

ipasphinx/setup.py

@@ -16,5 +16,5 @@ if __name__ == "__main__":
         doc=__doc__,
         package_dir={"ipasphinx": ""},
         packages=["ipasphinx"],
-        install_requires=["ipaserver", "ipalib", "sphinx", "m2r2"],
+        install_requires=["ipaserver", "ipalib", "sphinx > 3.0", "m2r2"],
     )