Upgrade virtualenv pip in Python docgen

Also ignore the docgen-generated directories under doc/.

doc/.gitignore

@@ -0,0 +1,2 @@
+build/
+src/

tools/distrib/python/docgen.py

@@ -71,6 +71,8 @@ environment.update({
 
 subprocess_arguments_list = [
     {'args': ['virtualenv', VIRTUALENV_DIR], 'env': environment},
+    {'args': [VIRTUALENV_PIP_PATH, 'install', '--upgrade', 'pip'],
+     'env': environment},
     {'args': [VIRTUALENV_PIP_PATH, 'install', '-r', REQUIREMENTS_PATH],
      'env': environment},
     {'args': [VIRTUALENV_PYTHON_PATH, SETUP_PATH, 'build'], 'env': environment},