Introduced Build Stages to our build process (celery#4429)

* Move lint to another build stage.

* Lint first, test later.

* Run flake8 with two jobs.

.travis.yml

@@ -9,6 +9,9 @@ python:
   - '3.6'
 os:
     - linux
+stages:
+  - lint
+  - test
 env:
   global:
   - PYTHONUNBUFFERED=yes
@@ -29,16 +32,22 @@ matrix:
     env: TOXENV=pypy-integration-dynamodb PYPY_VERSION="pypy2.7-5.8.0"
   - python: '3.5'
     env: TOXENV=flake8
+    stage: lint
   - python: '3.5'
     env: TOXENV=flakeplus
+    stage: lint
   - python: '3.5'
     env: TOXENV=apicheck
+    stage: lint
   - python: '3.5'
     env: TOXENV=configcheck
+    stage: lint
   - python: '3.5'
     env: TOXENV=pydocstyle
+    stage: lint
   - python: '3.5'
     env: TOXENV=isort-check
+    stage: lint
 before_install:
     - if [[ -v MATRIX_TOXENV ]]; then export TOXENV=${TRAVIS_PYTHON_VERSION}-${MATRIX_TOXENV}; fi; env
     - |

tox.ini

@@ -68,7 +68,7 @@ commands =
 
 [testenv:flake8]
 commands =
-    flake8 {toxinidir}/celery {toxinidir}/t
+    flake8 -j 2 {toxinidir}/celery {toxinidir}/t
 
 [testenv:flakeplus]
 commands =