Coerce pytest args to str to fix assertion error in pytest

This fixes a problem I encountered when running the tests with ``-t`` on an rst file.  E.g., ``python setup.py test -t coordinates/index.rst``  That should run the doctests for just that file, but instead, it yields the cryptic traceback I've included at the bottom of this message.  I traced the cause to a line deep in the bowels of pytest that does ``isinstance(arg, str)``, which fails for unicode arguments.  So this PR  just forces the arguments passed into pytest for setting plugins to be ``str``.  

```
running build_scripts
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "astropy/__init__.py", line 196, in test
    skip_docs=skip_docs)
  File "astropy/tests/helper.py", line 259, in run_tests
    result = pytest.main(args=all_args, plugins=plugins)
  File "_pytest.config", line 18, in main
  File "_pytest.config", line 62, in _prepareconfig
  File "_pytest.core", line 376, in __call__
  File "_pytest.core", line 387, in _docall
  File "_pytest.core", line 288, in execute
  File "_pytest.helpconfig", line 25, in pytest_cmdline_parse
  File "_pytest.core", line 288, in execute
  File "_pytest.config", line 617, in pytest_cmdline_parse
  File "_pytest.config", line 710, in parse
  File "_pytest.config", line 686, in _preparse
  File "_pytest.core", line 185, in consider_preparse
  File "_pytest.core", line 195, in consider_pluginarg
  File "_pytest.core", line 210, in import_plugin
AssertionError
```