build: fix hgrc manpage building with docutils 0.9

Since docutils 0.9, `roman` module has been moved from module directory root
(i.e. `site-packages/roman.py`) to `docutils.utils` module. Therefore `import
roman` statement should be wrapped in `try: ... except ImportError: ...` block
to handle importing correctly.

doc/hgmanpage.py

@@ -47,7 +47,10 b' by the command whatis or apropos.'
 import re
 
 from docutils import nodes, writers, languages
+try:
 import roman
+except ImportError:
+    from docutils.utils import roman
 import inspect
 
 FIELD_LIST_INDENT = 7