Enable devel testing job (#320)

* Enable devel testing job

Start testing pytest with code from pytest master and with
prereleases of other dependencies, allowing us to detect
breakages before upstream releases are made.

Author: ssbarnea

.travis.yml

@@ -1,5 +1,11 @@
 language: python
 jobs:
+  allow_failures:
+    - name: devel
+      python: 3.8
+      dist: xenial
+      env: TOXENV=devel
+
   include:
     - stage: tests
       language: node_js
@@ -41,6 +47,11 @@ jobs:
       sudo: required
       env: TOXENV=py37-ansi2html
 
+    - name: devel
+      python: 3.8
+      dist: xenial
+      env: TOXENV=devel
+
     -
       python: 3.8
       env: TOXENV=py38-ansi2html

tox.ini

@@ -21,6 +21,14 @@ basepython = python3
 deps = pre-commit
 commands = pre-commit run --all-files --show-diff-on-failure
 
+[testenv:devel]
+description = Tests with unreleased deps
+basepython = python3
+pip_pre = True
+deps =
+    {[testenv]deps}
+    pytest @ git+https://github.com/pytest-dev/pytest.git
+
 [flake8]
 max-line-length = 88
 exclude = .eggs,.tox