tox/Travis: use separate coverage factor

This will only install/run coverage on demand.

.travis.yml

@@ -9,46 +9,46 @@ matrix:
     python: 3.6
   - env: TOXENV=readme-py27
     python: 2.7
-  - env: TOXENV=py27-dj111
+  - env: TOXENV=py27-dj111-coverage
     python: 2.7
-  - env: TOXENV=py34-dj111
+  - env: TOXENV=py34-dj111-coverage
     python: 3.4
-  - env: TOXENV=py35-dj111
+  - env: TOXENV=py35-dj111-coverage
     python: 3.5
-  - env: TOXENV=py36-dj111
+  - env: TOXENV=py36-dj111-coverage
     python: 3.6
-  - env: TOXENV=pypy-dj111
+  - env: TOXENV=pypy-dj111-coverage
     python: pypy
-  - env: TOXENV=py34-dj20
+  - env: TOXENV=py34-dj20-coverage
     python: 3.4
-  - env: TOXENV=py35-dj20
+  - env: TOXENV=py35-dj20-coverage
     python: 3.5
-  - env: TOXENV=py35-djmaster
+  - env: TOXENV=py35-djmaster-coverage
     python: 3.5
-  - env: TOXENV=py36-dj20
+  - env: TOXENV=py36-dj20-coverage
     python: 3.6
-  - env: TOXENV=py36-dj21
+  - env: TOXENV=py36-dj21-coverage
     python: 3.6
-  - env: TOXENV=py36-djmaster
+  - env: TOXENV=py36-djmaster-coverage
     python: 3.6
-  - env: TOXENV=py37-dj20
+  - env: TOXENV=py37-dj20-coverage
     python: 3.7
     dist: xenial
     sudo: true
-  - env: TOXENV=py37-dj21
+  - env: TOXENV=py37-dj21-coverage
     python: 3.7
     dist: xenial
     sudo: true
-  - env: TOXENV=py37-djmaster
+  - env: TOXENV=py37-djmaster-coverage
     python: 3.7
     dist: xenial
     sudo: true
   allow_failures:
-  - env: TOXENV=py35-djmaster
+  - env: TOXENV=py35-djmaster-coverage
     python: 3.5
-  - env: TOXENV=py36-djmaster
+  - env: TOXENV=py36-djmaster-coverage
     python: 3.6
-  - env: TOXENV=py37-djmaster
+  - env: TOXENV=py37-djmaster-coverage
     python: 3.7
     dist: xenial
     sudo: true
@@ -57,7 +57,7 @@ install:
 script: tox -v
 after_success:
 - |
-  if [[ "$TOXENV" = py* ]]; then
+  if [[ "${TOXENV#-coverage}" != "$TOXENV" ]]; then
     pip install codecov
     coverage xml
     codecov --required -X gcov fix pycov -f coverage.xml --flags ${TOXENV//-/ }

tests/requirements.txt

@@ -1,4 +1,3 @@
-coverage
 django-discover-runner
 mock
 dj-database-url

tox.ini

@@ -16,7 +16,8 @@ usedevelop = true
 setenv =
     DJANGO_SETTINGS_MODULE = tests.settings.main
     DJANGO_CONFIGURATION = Test
-    COVERAGE_PROCESS_START = {toxinidir}/setup.cfg
+    coverage: COVERAGE_PROCESS_START = {toxinidir}/setup.cfg
+    coverage: _TEST_RUN_PREFIX=coverage run
 deps =
     -rtests/requirements.txt
     dj18: django>=1.8,<1.9
@@ -26,13 +27,14 @@ deps =
     dj21: django>=2.1a1,<2.2
     djmaster: https://github.com/django/django/archive/master.tar.gz#egg=django
     py27,pypy: mock
-    coverage_enable_subprocess
+    coverage: coverage
+    coverage: coverage_enable_subprocess
 
 commands =
     python --version
-    coverage run {envbindir}/django-cadmin test -v2 {posargs:tests}
-    coverage combine . tests/docs
-    coverage report -m --skip-covered
+    {env:_TEST_RUN_PREFIX:} {envbindir}/django-cadmin test -v2 {posargs:tests}
+    coverage: coverage combine . tests/docs
+    coverage: coverage report -m --skip-covered
 
 [testenv:readme-py27]
 commands = python setup.py check -r -s