fix building docs on Debian Bullseye (ganeti#1602)

When building docs with man pages (`--enable-manpages-in-doc`) the
following issues exists:

* Sphinx refdomain=None no longer works. In a previous fix (commit
  388c0b6) refdomain=None was
  introduced. However this emits a deprecation warning on pre Debian
  Bullseye (i.e. Ubuntu Focal) and no longer works on Bullseye. Fix this
  by setting refdomain=std:

  RemovedInSphinx20Warning: Invalid pendig_xref node detected. :doc: reference should have refdomain=std attribute.

* On Bullseye an other deprecation warning is emitted and no longer
  works the old way. Fixing this as suggested in the warning, while keeping
  support for sphinx before 2.1.0:

  RemovedInSphinx40Warning: app.add_lexer() API changed; Please give lexer class instead of instance

Signed-off-by: Sascha Lucas <sascha_lucas@web.de>

lib/build/shell_example_lexer.py

@@ -41,6 +41,7 @@
 
 from pygments.lexer import RegexLexer, bygroups, include
 from pygments.token import Name, Text, Generic, Comment
+import sphinx
 
 
 class ShellExampleLexer(RegexLexer):
@@ -79,4 +80,8 @@ class ShellExampleLexer(RegexLexer):
 
 
 def setup(app):
-  app.add_lexer("shell-example", ShellExampleLexer())
+  version = tuple(map(int, sphinx.__version__.split('.')))
+  if version >= (2, 1, 0):
+    app.add_lexer("shell-example", ShellExampleLexer)
+  else:
+    app.add_lexer("shell-example", ShellExampleLexer())

lib/build/sphinx_ext.py

@@ -360,7 +360,7 @@ def _ManPageNodeClass(*args, **kwargs):
 
   # Force custom title
   kwargs["refexplicit"] = True
-  kwargs["refdomain"] = None
+  kwargs["refdomain"] = "std"
 
   return sphinx.addnodes.pending_xref(*args, **kwargs)