Only use old virtualenv with old tox

Fixes #74

setup.py

@@ -27,7 +27,6 @@ def long_description():
             "pytest",
             "pytest-xdist",
             "packaging",
-            "virtualenv<20.22",  # pin required for tests with Python 3.6 envs
         ],
     },
     python_requires=">=3.6",

tests/conftest.py

@@ -2,7 +2,7 @@
 import shutil
 
 import pytest
-from utils import FIXTURES_DIR, TOX4
+from utils import FIXTURES_DIR, TOX4, modify_config, drop_unsupported_pythons
 
 
 @pytest.fixture(autouse=True)
@@ -11,6 +11,9 @@ def projdir(tmp_path, monkeypatch, worker_id):
     pwd.mkdir()
     for fname in "tox.ini", "setup.py", "pyproject.toml":
         shutil.copy(FIXTURES_DIR / fname, pwd)
+    if TOX4:
+        with modify_config(pwd / "tox.ini") as config:
+            config["tox"]["envlist"] = drop_unsupported_pythons(config["tox"]["envlist"])
     monkeypatch.chdir(pwd)
     # https://github.com/pypa/pip/issues/5345#issuecomment-386424455
     monkeypatch.setenv("XDG_CACHE_HOME",

tests/utils.py

@@ -83,11 +83,17 @@ def is_available(python):
     return True
 
 
+@functools.lru_cache()
+def drop_unsupported_pythons(envlist):
+    return envlist[len("py36,py37,"):] if TOX4 else envlist
+
+
 @functools.lru_cache()
 def envs_from_tox_ini():
     cp = ConfigParser()
     cp.read(FIXTURES_DIR / "tox.ini")
-    return cp["tox"]["envlist"].split(",")
+    envlist = drop_unsupported_pythons(cp["tox"]["envlist"])
+    return envlist.split(",")
 
 
 def tox_footer(envs=None, spaces=8):

tox.ini

@@ -9,6 +9,7 @@ extras =
     tests
 deps=
     tox3: tox < 4
+    tox3: virtualenv < 20.22
     tox4: tox >=4.0.15,< 5
 commands =
     pytest -v {posargs} tests