ci: add a job for min supported pytest version

Make sure we don't break the promise.

.github/workflows/test.yml

@@ -34,6 +34,7 @@ jobs:
       fail-fast: false
       matrix:
         tox_env:
+          - "py38-pytestmin"
           - "py38-pytestlatest"
           - "py39-pytestlatest"
           - "py310-pytestlatest"
@@ -45,6 +46,8 @@ jobs:
 
         os: [ubuntu-latest, windows-latest]
         include:
+          - tox_env: "py38-pytestmin"
+            python: "3.8"
           - tox_env: "py38-pytestlatest"
             python: "3.8"
           - tox_env: "py39-pytestlatest"

tox.ini

@@ -9,6 +9,7 @@ isolated_build = true
 [testenv]
 extras = testing
 deps =
+  pytestmin: pytest==6.2.0
   pytestlatest: pytest
   pytestmain: git+https://github.com/pytest-dev/pytest.git
 commands=