requirements: Explicitly install pygments 2.3.1 for pudb on py 3.4.

The latest pygments release depends on py3.5+, so when pudb installs the
latest version, this breaks our dev installs on py3.4.

This commit is a workaround for this issue, to enable pipenv dev
installs, which are currently also used for testing in CI.

Author: sumanthvrao

Pipfile

@@ -18,6 +18,7 @@ pytest = "==3.4.2"
 pytest-pep8 = "==1.0.6"
 pytest-mock = "==1.7.1"
 pytest-cov = "==2.5.1"
+pygments = "==2.3.1"  # For pudb, python 3.4
 pudb = "==2017.1.4"
 tornado = "~=5.1"  # For snakeviz, python 3.4+
 snakeviz = "==0.4.2"

requirements.txt

@@ -5,6 +5,7 @@ pytest==3.4.2
 pytest-cov==2.5.1
 pytest-pep8==1.0.6
 pytest-mock==1.7.1
+pygments==2.3.1
 pudb==2017.1.4
 tornado~=5.1
 snakeviz==0.4.2

setup.py

@@ -37,6 +37,7 @@ def long_description():
 
 dev_helper_deps = [
     'mypy==0.641',
+    'pygments==2.3.1',
     'pudb==2017.1.4',
     'tornado~=5.1',
     'snakeviz==0.4.2',