Update CI to use Python 3.8 through 3.10 (#386)

* Try to build on Python 3.8 and 3.9

* Delete .travis.yml

* Update ci.yaml

* Don't duplicate CI runs on push and pull_request

* Update Python versions following NEP 29

* Unpin actions versions

* Include version targets as they are required

* Turn off macOS + Python 3.10 tests

* Cleanup

.github/workflows/ci.yaml

@@ -11,13 +11,17 @@ jobs:
       fail-fast: false
       matrix:
         os: [macOS-latest, ubuntu-latest]
-        python-version: [3.6, 3.7]
+        python-version: ["3.8", "3.9", "3.10"]
+        exclude:
+          # These OpenMM builds are broken as of 8/2022
+          - os: macOS-latest
+            python-version: "3.10"
     env:
       CI_OS: ${{ matrix.os }}
       PYVER: ${{ matrix.python-version }}
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
 
     - uses: conda-incubator/setup-miniconda@v2
       with: