BLD: pin CI Ubuntu version to 20.04

Fixes half of #10.

.github/workflows/main.yml

@@ -4,16 +4,15 @@ name: pyfastg CI
 on: [push, pull_request]
 jobs:
   build:
-    runs-on: ubuntu-latest
+    # Stopgap solution -- "ubuntu-latest" (22.04, as of writing) no longer
+    # supports Python 3.6. See https://github.com/fedarko/pyfastg/issues/10.
+    runs-on: ubuntu-20.04
 
     strategy:
       matrix:
         # Gotta specify 3.10 as a string to avoid it being converted to 3.1:
         # https://github.com/actions/setup-python/issues/160
-        # The -dev after 3.11 is so that we can test on the latest dev version
-        # of python 3.11, since I believe no stable version of 3.11 exists yet
-        # (see https://github.com/actions/setup-python/issues/160 again)
-        python-version: [3.6, 3.7, 3.8, 3.9, "3.10", "3.11-dev"]
+        python-version: [3.6, 3.7, 3.8, 3.9, "3.10", "3.11"]
 
     steps: