Install all required packages during docs building

Fix missing plugin documentation by installing everything.
Include extra system package requirements as well.
Remove Mock as it's not necessary any more.

.readthedocs.yaml

@@ -1,7 +1,12 @@
 # Config for building https://did.readthedocs.io/
 version: 2
+
 python:
     install:
       - method: pip
         path: .
-        extra_requirements: [docs]
+        extra_requirements: [all]
+build:
+    apt_packages:
+      - python3-dev
+      - libkrb5-dev

docs/conf.py

@@ -30,19 +30,6 @@
         # and fall back to 'default' if neither of those are available
         HTML_THEME = 'default'
 
-# Mock C modules (needed for jira and rt plugins)
-from mock import Mock as MagicMock
-
-
-class Mock(MagicMock):
-    @classmethod
-    def __getattr__(cls, name):
-        return Mock()
-
-
-MOCK_MODULES = ['gssapi', 'requests_gssapi', 'bugzilla']
-sys.modules.update((mod_name, Mock()) for mod_name in MOCK_MODULES)
-
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.