Run tests in parallel with pytest-xdist

Also stop running tox in parallel when not required because it buffers
output, making it harder to see what is happening.

Output could be shown unbuffered with --parallel-live but it isn't
really helpful for what we're trying to achieve.

.travis.yml

@@ -8,38 +8,45 @@ addons:
 jobs:
   include:
     - python: &python_main_version 3.8
-      env: TOXENV=docs,black,flake8,isort,mypy,pylint
+      env:
+        TOXENV: docs,black,flake8,isort,mypy,pylint
+        TOX_PARALLEL: "-p"
     - python: 3.6
-      env: TOXENV=py36
+      env:
+        TOXENV: py36
     - python: 3.7
-      env: TOXENV=py37
+      env:
+        TOXENV: py37
     - python: 3.8
-      env: TOXENV=py38
+      env:
+        TOXENV: py38
     - python: 3.9
-      env: TOXENV=py39
+      env:
+        TOXENV: py39
     - python: pypy3
-      env: TOXENV=pypy3
+      env:
+        TOXENV: pypy3
     - os: osx
       # Not required but makes Travis' output prettier
       language: generic
       env:
-        - MB_PYTHON_VERSION=3.6
-        - TOXENV=py36
+        MB_PYTHON_VERSION: 3.6
+        TOXENV: py36
     - os: osx
       language: generic
       env:
-        - MB_PYTHON_VERSION=3.7
-        - TOXENV=py37
+        MB_PYTHON_VERSION: 3.7
+        TOXENV: py37
     - os: osx
       language: generic
       env:
-        - MB_PYTHON_VERSION=3.8
-        - TOXENV=py38
+        MB_PYTHON_VERSION: 3.8
+        TOXENV: py38
     - os: osx
       language: generic
       env:
-        - MB_PYTHON_VERSION=3.9
-        - TOXENV=py39
+        MB_PYTHON_VERSION: 3.9
+        TOXENV: py39
 before_install: |
   if [ $TRAVIS_OS_NAME == osx ]; then
     git clone https://github.com/matthew-brett/multibuild
@@ -62,7 +69,7 @@ before_install: |
     popd
   fi
   pip install tox
-script: tox -p
+script: "tox $TOX_PARALLEL"
 notifications:
   email:
     on_success: change
@@ -89,3 +96,4 @@ env:
     - LIBZEN_VERSION: 0.4.38
     - UBUNTU_VERSION: 18.04
     - PYTHON_MAIN_VERSION: *python_main_version
+    - TOX_PARALLEL_NO_SPINNER: 1

appveyor.yml

@@ -35,7 +35,7 @@ install:
     - "move MediaInfo.dll pymediainfo"
     - "move Developers\\License.html docs"
     # TODO: remove the constraint when https://github.com/tox-dev/tox/issues/1550 is fixed
-    - "pip install --upgrade setuptools pytest tox==3.8.3 twine wheel"
+    - "pip install --upgrade setuptools tox==3.8.3 twine wheel"
 build_script:
   - "python setup.py bdist_wheel"
 test_script:

tox.ini

@@ -14,9 +14,10 @@ envlist =
 [testenv]
 deps =
   pytest
+  pytest-xdist
   setuptools_scm
 commands =
-  pytest {posargs}
+  pytest -n auto {posargs}
 
 [testenv:docs]
 deps = sphinx