highlight: put pygments import inside demandimport.deactivated

I tripped on some weirdness relating to _thread vs threading way down
in a dep of highlight recently. I'm not really sure why I'm only just
seeing this defect now, but experimentally this fixes the problem, and
shouldn't cause any load-time slowness for people until pygments is
actually about to be used since highlight.highlight is still lazily
loaded in the highlight/__init__.py file.

hgext/highlight/highlight.py

@@ -10,11 +10,6 b''
 
 from __future__ import absolute_import
 
-import pygments
-import pygments.formatters
-import pygments.lexers
-import pygments.util
-
 from mercurial import demandimport
 demandimport.ignore.extend(['pkgutil', 'pkg_resources', '__main__'])
 
@@ -23,6 +18,12 b' from mercurial import ('
     util,
 )
 
+with demandimport.deactivated():
+    import pygments
+    import pygments.formatters
+    import pygments.lexers
+    import pygments.util
+
 highlight = pygments.highlight
 ClassNotFound = pygments.util.ClassNotFound
 guess_lexer = pygments.lexers.guess_lexer