Properly install Pattern library for documentation build (#2626)

* Probably fixes #2534

* Uppercase P

* Added comment

setup.py

@@ -276,13 +276,14 @@ def finalize_options(self):
  ])
 
 if (3, 0) < sys.version_info < (3, 7):
- linux_testenv.extend(['nmslib']) 
- 
+ linux_testenv.extend(['nmslib'])
+
 docs_testenv = linux_testenv + distributed_env + [
  'sphinx',
  'sphinxcontrib-napoleon',
  'plotly',
- 'pattern <= 2.6',
+# Pattern's version is specified to install Pattern 3.6, which adds python3 support
+ 'Pattern >= 3.6',
  'sphinxcontrib.programoutput',
 ]
 #
@@ -360,7 +361,7 @@ def finalize_options(self):
 
  url='http://radimrehurek.com/gensim',
  download_url='http://pypi.python.org/pypi/gensim',
- 
+
  license='LGPLv2.1',
 
  keywords='Singular Value Decomposition, SVD, Latent Semantic Indexing, '