Make docsrc/exts/sphinxlocal/builders/manpage.py compilable.

The file docsrc/exts/sphinxlocal/builders/manpage.py
extends the file sphinx/builders/manpage.py from the Sphinx 1.4
source. The origin contains

from sphinx.environment import NoUri

and its code does raise the NoUri exception. Later versions of
Sphinx do not contain this line in sphinx/builders/manpage.py .

Per https://www.sphinx-doc.org/en/master/extdev/deprecated.html
sphinx.environment.NoUri is deprecated and shall be replaced
by sphinx.errors.NoUri. Sphinx 1.4 does not have sphinx.errors.NoUri.

While inteded for removal in 4.0, sphinx.environment.NoUri was
in fact removed in a previous version: https://github.com/sphinx-doc/sphinx/issues/9032

That said:

• docsrc/exts/sphinxlocal/builders/manpage.py does not use sphinx.environment.NoUri, but contains it because of copy-paste oparations from the super class.

• Sphinx 3.x does not contain sphinx.environment.NoUri neither.