Fix formatting in contribution function equation (#283)

* type on g_of_t equation formatting

* move pytest import to avoid needing it in docs build

fiasco/ions.py

@@ -709,7 +709,7 @@ def contribution_function(self, density: u.cm**(-3), **kwargs) -> u.cm**3 * u.er
 
         .. math::
 
-           G_{ij} = mathrm{Ab}(X)f_{X,k}N_jA_{ij}\Delta E_{ij}\frac{1}{n_e},
+           G_{ij} = \mathrm{Ab}(X)f_{X,k}N_jA_{ij}\Delta E_{ij}\frac{1}{n_e},
 
         Note that the contribution function is often defined in differing ways by different authors.
         The contribution function is defined as above in :cite:t:`young_chianti_2016`.

fiasco/tests/idl/helpers.py

@@ -2,7 +2,6 @@
 Helpers for tests related to comparing with IDL output
 """
 import pathlib
-import pytest
 
 from astropy.utils.data import get_pkg_data_path
 
@@ -61,6 +60,8 @@ def run_idl_script(idl_env, script, input_args, save_vars, file_name, version, f
     file_path = get_idl_test_output_filepath(file_name, version)
     if not file_path.is_file():
         if idl_env is None:
+            # Import here so that this can be used without a hard pytest dependency
+            import pytest
             pytest.skip("""To run the IDL comparison tests, you must:
                             1. Specify a path to a working IDL executable,
                             2. Specify a path to the CHIANTI IDL code,