doc: migrate to m2r2 and newer sphinx, add plantuml to venv

m2r project was forked to m2r2 which is actively developed.
m2r2 works with new Sphinx versions.

Update our list of documentation requirements and add support for
plantuml to be able to integrate diagrams.

Fixes: https://pagure.io/freeipa/issue/9148

Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Francisco Trivino <ftrivino@redhat.com>

doc/conf.py

@@ -40,7 +40,8 @@ extensions = [
     'sphinx.ext.autodoc',
     'sphinx.ext.intersphinx',
     'sphinx.ext.viewcode',
-    'm2r',
+    'sphinxcontrib.plantuml',
+    'm2r2',
 ]
 
 # Add any paths that contain templates here, relative to this directory.

doc/requirements.txt

@@ -1,8 +1,11 @@
-## m2r is not compatible with Sphinx 3.x yet
-sphinx >2.1,<3.0
+wheel
 
-## convert markdown to rest
-m2r
+sphinx
+
+# convert markdown to rest
+# m2r2 fork is active in development and works with
+# newer sphinx versions
+m2r2
 
 ## ipa dependencies
 dnspython
@@ -26,3 +29,7 @@ lxml
 
 ## No sufficient PyPI packages
 # dogtag-pki
+
+# Sequence diagrams generator
+sphinxcontrib-plantuml
+pillow