Modify CI test matrix to test on Numpy 1.17 and 1.20, Python 3.7 and 3.9

Test only on minimum and maximum supported versions
according to NEP. I.e. minimum of NumPy 1.17 and Py3.7,
maximum of NumPy 1.20 and Py3.9. This drops testing on
the middle Python 3.8 version to conserve CI resources.

.github/workflows/ci_tests.yaml

@@ -30,7 +30,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.7, 3.8, 3.9]
+        numpy-version: [1.17, 1.20]
+        python-version: [3.7, 3.9]
         os: [ubuntu-latest, macOS-latest, windows-latest]
         # Is it a draft Pull Request (true or false)?
         isDraft:
@@ -44,9 +45,6 @@ jobs:
           - os: ubuntu-latest
             python-version: 3.7
             isDraft: true
-          - os: ubuntu-latest
-            python-version: 3.8
-            isDraft: true
     defaults:
       run:
         shell: bash -l {0}
@@ -82,7 +80,8 @@ jobs:
       # Install GMT and other required dependencies from conda-forge
       - name: Install dependencies
         run: |
-          conda install gmt=6.1.1 numpy pandas xarray netCDF4 packaging \
+          conda install gmt=6.1.1 numpy=${{ matrix.numpy-version }} pandas \
+                        xarray netCDF4 packaging \
                         codecov coverage[toml] ipython make \
                         pytest-cov pytest-mpl pytest>=6.0 \
                         sphinx-gallery