Add pytest-4 to the tests

Author: marv

.appveyor.yml

@@ -4,14 +4,18 @@ environment:
   - TOXENV: "py27-pytest29"
   - TOXENV: "py27-pytest30"
   - TOXENV: "py27-pytest31"
+  - TOXENV: "py27-pytest4"
   - TOXENV: "py35-pytest29"
   - TOXENV: "py35-pytest30"
   - TOXENV: "py35-pytest31"
+  - TOXENV: "py35-pytest4"
   - TOXENV: "py36-pytest29"
   - TOXENV: "py36-pytest30"
   - TOXENV: "py36-pytest31"
+  - TOXENV: "py36-pytest4"
   - TOXENV: "py37-pytest30"
   - TOXENV: "py37-pytest31"
+  - TOXENV: "py37-pytest4"
 
 install:
   - echo Installed Pythons

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 # Skip py37-pytest29 as such a combination does not work (#192)
-envlist=py{27,34,35,36}-pytest{29,30,31},py37-pytest{30,31}
+envlist=py{27,34,35,36}-pytest{29,30,31,4},py37-pytest{30,31,4}
 
 [testenv]
 commands=
@@ -11,6 +11,7 @@ deps=
   pytest29: pytest~=2.9.0
   pytest30: pytest~=3.0.0
   pytest31: pytest~=3.1.0
+  pytest4: pytest~=4.6.9
 
 [testenv:py27-xdist]
 basepython=python2.7