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

1 files changed, 6 insertions, 0 deletions

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,