fix: changes for PyPy3.8

- Update tox.ini to let us run against PyPy3.8

- Some 3.8 behavior is (apparently) only on CPython

- PyPy3.8 doesn't get along with virtualenv yet
  (pypa/virtualenv#2182), so use venv instead for our
  virtualenv tests.

coverage/env.py

@@ -77,7 +77,7 @@ class PYBEHAVIOR:
     # When a function is decorated, does the trace function get called for the
     # @-line and also the def-line (new behavior in 3.8)? Or just the @-line
     # (old behavior)?
-    trace_decorated_def = (PYVERSION >= (3, 8))
+    trace_decorated_def = (CPYTHON and PYVERSION >= (3, 8))
 
     # Are while-true loops optimized into absolute jumps with no loop setup?
     nix_while_true = (PYVERSION >= (3, 8))

tests/test_oddball.py

@@ -148,6 +148,7 @@ class MemoryLeakTest(CoverageTest):
     """
     @flaky
     @pytest.mark.skipif(env.JYTHON, reason="Don't bother on Jython")
+    @pytest.mark.skipif(not env.C_TRACER, reason="Only the C tracer has refcounting issues")
     def test_for_leaks(self):
         # Our original bad memory leak only happened on line numbers > 255, so
         # make a code object with more lines than that.  Ugly string mumbo

tests/test_process.py

@@ -1691,7 +1691,7 @@ def venv_world_fixture(tmp_path_factory):
     venv_world = tmp_path_factory.mktemp("venv_world")
     with change_dir(venv_world):
         # Create a virtualenv.
-        run_command("python -m virtualenv venv")
+        run_command("python -m venv venv")
 
         # A third-party package that installs a few different packages.
         make_file("third_pkg/third/__init__.py", """\

tox.ini

@@ -28,7 +28,7 @@ install_command = python -m pip install -U {opts} {packages}
 
 passenv = *
 setenv =
-    pypy,pypy3: COVERAGE_NO_CTRACER=no C extension under PyPy
+    pypy{3,37,38}: COVERAGE_NO_CTRACER=no C extension under PyPy
     jython: COVERAGE_NO_CTRACER=no C extension under Jython
     jython: PYTEST_ADDOPTS=-n 0
     # For some tests, we need .pyc files written in the current directory,