Fix test failure when h5py and matplotlib are not installed

[smaret]

h5py and matplotlib are optional dependencies of astropy. When these are not installed, reproject tests fail with the following error:

reproject/conftest.py:15: in <module>
    del PYTEST_HEADER_MODULES['h5py']
E   KeyError: 'h5py'

This PR fixes the tests.

[smaret]

I don't understand why codecov complains. conftest.py is excluded from the coverage tests in reproject/tests/coveragerc:

[run]
source = {packagename}
omit =
   {packagename}/_astropy_init*
   {packagename}/conftest*

[astrofrog]

This should now be fixed in master:

reproject/reproject/conftest.py

Lines 23 to 25 in 6d2119b

	PYTEST_HEADER_MODULES.pop('Pandas', None)
	PYTEST_HEADER_MODULES.pop('h5py', None)
	PYTEST_HEADER_MODULES.pop('Matplotlib', None)

so I'll close this - thanks for the PR!