ci: attempt to fix github release CI

Github release CI was running on ubuntu 18.04 withh python 3.6.
Installing tomli==2.0.1, which is required in dev, triggers a failure in
python 3.6 because it is no longer available.

.github/workflows/release.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-18.04
+          - os: ubuntu-latest
             locale: C.UTF-8
           - os: macos-10.15
             locale: en_US.UTF-8
@@ -23,7 +23,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: 3.6
+          python-version: 3.7
       - name: Upgrade pip
         run: python -m pip install --upgrade pip setuptools==44.0.0
       - name: Print info about the current python installation