docs/conf.py: Add .venv dir to exclude_patterns.

It's useful to try different Sphinx versions using virtualenv/venv, so
exclude a common venv dir name from Sphinx processing.

docs/conf.py

@@ -113,7 +113,7 @@ release = '1.9.1'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
-exclude_patterns = ['build']
+exclude_patterns = ['build', '.venv']
 
 # The reST default role (used for this markup: `text`) to use for all
 # documents.