Add tomli as a dependency in GMT Dev Tests (GenericMappingTools#1401)

Adding tomli to fix the GMT Dev Tests breakages reported
in GenericMappingTools#1392. This is a temporary measure until `pytest-cov`
releases a new version > 2.12.1.

.github/workflows/ci_tests_dev.yaml

@@ -86,7 +86,8 @@ jobs:
           conda install ninja cmake libblas libcblas liblapack fftw gdal geopandas \
                         ghostscript libnetcdf hdf5 zlib curl pcre make dvc
           pip install --pre numpy pandas xarray netCDF4 packaging \
-                            ipython pytest-cov pytest-mpl pytest>=6.0 sphinx-gallery
+                            ipython pytest-cov pytest-mpl pytest>=6.0 sphinx-gallery \
+                            tomli
 
       # Build and install latest GMT from GitHub
       - name: Install GMT ${{ matrix.gmt_git_ref }} branch (Linux/macOS)