add own ci job for legacy setuptools testing an skip it for normal tests

.github/workflows/python-tests.yml

@@ -49,6 +49,17 @@ jobs:
         if: matrix.os == 'ubuntu-latest'
       - run: pytest
 
+  test_legacy_setuptools:
+    runs-on: ubuntu-lates
+    steps:
+      - uses: actions/checkout@v1
+      - name: Setup python
+        uses: actions/setup-python@v2
+        with:
+          python-version: "3.6"
+          architecture: x64
+      - run: pip install -e .[toml] pytest virtualenv
+      - run: pytest --test-legacy testing/test_setuptools_support.py
   check_selfinstall:
     runs-on: ubuntu-latest
     strategy:

testing/conftest.py

@@ -20,6 +20,13 @@ def pytest_report_header():
     return res
 
 
+def pytest_addoption(parser):
+    group = parser.getgroup("setuptools_scm")
+    group.addoption(
+        "--test-legacy", dest="scm_test_virtualenv", default=False, action="store_true"
+    )
+
+
 class Wd:
     commit_command = None
     add_command = None

testing/test_setuptools_support.py

@@ -59,6 +59,10 @@ def get_venv(self, python, pip, setuptools, prefix="scm"):
 
 @pytest.fixture
 def venv_maker(pytestconfig):
+    if not pytestconfig.getoption("--test-legacy"):
+        pytest.skip(
+            "testing on legacy setuptools disabled, pass --test-legacy to run them"
+        )
     dir = pytestconfig.cache.makedir("setuptools_scm_venvs")
     path = pathlib.Path(str(dir))
     return VenvMaker(path)