Improve CI / testing (#754)

* Use "testing" extras_require

This centralized testing requirements and allows for
`pip install -e .[testing]` etc.

pytidylib is kept separate according to
https://github.com/Python-Markdown/markdown/pull/754/files#r233561159.

* Travis: simplify installation of deps

* Travis: use xenial by default, trusty for py34

* Travis: codecov: use flags

* Travis: only install aspell for checkspelling job

* Remove coverage version restriction

4 files changed, 21 insertions, 18 deletions

diff --git a/.travis.yml b/.travis.yml
index c050ace..b0daef7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,4 @@
+dist: xenial
 sudo: false
 language: python
 
@@ -7,39 +8,42 @@ matrix:
       env: TOXENV=py27
     - python: '3.4'
       env: TOXENV=py34
+      dist: trusty
     - python: '3.5'
       env: TOXENV=py35
     - python: '3.6'
       env: TOXENV=py36
     - python: '3.7'
       env: TOXENV=py37
-      dist: xenial
-      sudo: true
     - python: 'pypy'
       env: TOXENV=pypy
+      dist: trusty
     - python: 'pypy3'
       env: TOXENV=pypy3
+      dist: trusty
     - env: TOXENV=flake8
     - env: TOXENV=checkspelling
+      addons:
+        apt:
+          packages:
+          - aspell
+          - aspell-en
 
 addons:
   apt:
     packages:
     - libtidy-0.99-0
-    - aspell
-    - aspell-en
 
 install:
-  - pip install 'setuptools>=36'
-  - pip install tox
-  - pip install 'coverage<4.0'
-  - pip install codecov
+  # NOTE: setuptools needs to be installed explicitly for py34 (trusty).
+  - pip install 'setuptools>=36' tox
 
 script:
   - tox
 
 after_success:
-  - codecov
+  - pip install codecov
+  - codecov --flags "$TOXENV"
 
 branches:
   only:
diff --git a/setup.py b/setup.py
index 938defa..725b005 100755
--- a/setup.py
+++ b/setup.py
@@ -78,6 +78,12 @@ setup(
     packages=['markdown', 'markdown.extensions'],
     python_requires='>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*',
     install_requires=['setuptools >= 36'],
+    extras_require={
+        'testing': [
+            'coverage',
+            'pyyaml',
+        ],
+    },
     entry_points={
         'console_scripts': [
             '%s = markdown.__main__:run' % SCRIPT_NAME,
diff --git a/test-requirements.txt b/test-requirements.txt
deleted file mode 100644
index 0e8680f..0000000
--- a/test-requirements.txt
+++ /dev/null
@@ -1,5 +0,0 @@
-coverage<4.0
-pyyaml
-pytidylib
-mkdocs>=1.0
-mkdocs-nature
diff --git a/tox.ini b/tox.ini
index 0e130ae..ee8f50f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -4,10 +4,8 @@ requires = setuptools>=36
 isolated_build = True
 
 [testenv]
-deps =
-    coverage<4.0
-    pyyaml
-    pytidylib
+extras = testing
+deps = pytidylib
 commands = coverage run --source=markdown -m unittest discover {toxinidir}/tests
            coverage report --show-missing