Pin actions to Ubuntu 20.04 (#590)

The pointer for `ubuntu-latest` was updated to 22.04 on Oct 3
(see actions/runner-images#6399).

We still run tests on 2.7 and 3.6 and should deprecate support separately.

.github/workflows/push.yml

@@ -18,7 +18,7 @@ jobs:
           - '3.8'
           - '3.9'
           - '3.10'
-        os: [ ubuntu-latest ]
+        os: [ ubuntu-20.04 ]
     env:
       OS: ${{ matrix.os }}
       PYTHON_VERSION: ${{ matrix.python-version }}
@@ -60,7 +60,7 @@ jobs:
           env_vars: OS,PYTHON_VERSION
 
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version:
@@ -125,7 +125,7 @@ jobs:
 
   compat:
     name: Public API compatibility
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2

.github/workflows/tag.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   tag:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2

dev-requirements-py3.txt

@@ -1,2 +1,13 @@
 -r ./dev-requirements.txt
 prospector[with_pyroma]
+
+# More recent versions of these linters introduce rules that make it fail.
+# Pin these versions to keep the ruleset fixed.
+dodgy==0.2.1
+flake8==4.0.1
+flake8-polyfill==1.0.2
+mccabe==0.6.1
+prospector==1.7.7
+pycodestyle==2.8.0
+pyflakes==2.4.0
+pylint==2.13.9