From 9f4cc74b9a8fa5d4e503faf849428b0fb96ab0f7 Mon Sep 17 00:00:00 2001
From: matejcik <ja@matejcik.cz>
Date: Fri, 18 May 2018 10:58:28 +0200
Subject: [PATCH] travis: set virtualenv version to restore build on py33

---
 .travis.yml | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index 5658f9c3e8..57264da624 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -26,8 +26,9 @@ python:
 
 install:
     # Optimisation: build requirements as wheels, which get cached by Travis
-    - pip install "pip>=9.0" wheel
-    - pip install "setuptools>=38"
+    - pip install "pip>=9.0" wheel  # pip 9.0 understands `python_requires` constraints
+    - pip install "setuptools>=38"  # setuptools >= 38 are capable of using prebuilt wheels
+    - pip install "virtualenv<16.0.0"  # virtualenv 16.0.0 drops support for py33 without properly declaring it
     - pip install tox-travis
     - pip install flake8
     # protobuf-related dependencies