TST: attempt to use mamba instead of conda for GHW

for some reason, all of the python 3.6 builds are taking forever
specifically on the steps that install wotlpot and its pip
dependencies. (other python versions >= 3.7 all finish quickly.)
i suspect that conda is the problem -- will try to fix...

.github/workflows/main.yml

@@ -22,9 +22,8 @@ jobs:
       - name: Check out code
         uses: actions/checkout@v3
 
-      # https://github.com/conda-incubator/setup-miniconda#example-3-other-options
       - name: Install conda dependencies
-        uses: conda-incubator/setup-miniconda@v2
+        uses: mamba-org/setup-micromamba@v1
         with:
           activate-environment: wotplot
           environment-file: environment.yml
@@ -42,13 +41,13 @@ jobs:
       # practice, according to
       # https://github.com/marketplace/actions/setup-miniconda#environment-activation).
       - name: Install wotplot (and pip dependencies)
-        run: conda run -n wotplot pip install -e .[dev,viz]
+        run: pip install -e .[dev,viz]
 
       - name: Lint and stylecheck
-        run: conda run -n wotplot make stylecheck
+        run: make stylecheck
 
       - name: Run tests
-        run: conda run -n wotplot make test
+        run: make test
 
       - name: Upload code coverage information to Codecov
         uses: codecov/codecov-action@v2