add support for fits: write test FITS file in setUp

tests/test_fits.py

@@ -30,7 +30,7 @@ class TestFitsHeaderOps(unittest.TestCase):
                 fits.ImageHDU(data=np.random.randn(5, 5)),
             ]
         )
-        hl.writeto(test_fits_file)
+        hl.writeto(test_fits_file, overwrite=True)
 
     def test_fits_open(self):
         hl = fits.open(test_fits_file)