Change test coverage dependency  

I think https://github.com/astropy/astropy/pull/2112 changes this from ``pytest-cov`` to ``coverage``
Merge pull request #2212 from mwcraig/patch-4

Change test coverage dependency
Ensure name-less columns can be printed, plus test case and bug-fix note.
Don't generate .pyc files when running tests
str -> six.text_type, move change to 0.3.2 bug-fix section
Merge pull request #2214 from mdboom/misc/dont-write-pyc

Don't generate .pyc files when running tests
Merge pull request #2213 from mhvk/column-no-name-header

Column with no name cannot be printed
Update development_workflow.rst

typo fix
Merge pull request #2220 from larrybradley/patch-1

Update development_workflow.rst
Change test coverage dependency

I think https://github.com/astropy/astropy/pull/2112 changes this from ``pytest-cov`` to ``coverage``

Don't generate .pyc files when running tests

Ensure name-less columns can be printed, plus test case and bug-fix note.

str -> six.text_type, move change to 0.3.2 bug-fix section

Update development_workflow.rst

typo fix

Merge pull request #2197 from QuanTakeuchi/master

Issue with formatting coordinates with colons #2183

WCS accepts coordinates objects

modified WCS accepts coordinate objects

minor spaces removal