build: remove some unneeded tox steps

Bernát Gábor pointed out that we don't need the pip installation steps
because tox will have already installed the project.

tox.ini

@@ -9,9 +9,9 @@ skip_missing_interpreters = {env:COVERAGE_SKIP_MISSING_INTERPRETERS:True}
 toxworkdir = {env:TOXWORKDIR:.tox}
 
 [testenv]
-usedevelop = True
 extras =
     toml
+package = editable
 
 # PYVERSIONS
 deps =
@@ -30,15 +30,11 @@ setenv =
     # so override any local setting.
     PYTHONPYCACHEPREFIX=
 
-# $set_env.py: COVERAGE_PIP_ARGS - Extra arguments for `pip install`
-# `--no-build-isolation` will let tox work with no network.
 commands =
     # Create tests/zipmods.zip
     python igor.py zip_mods
 
-    # Build the C extension and test with the CTracer
-    python setup.py --quiet build_ext --inplace
-    python -m pip install {env:COVERAGE_PIP_ARGS} -q -e .
+    # Test with the CTracer that normal installation gives us.
     python igor.py test_with_tracer c {posargs}
 
     # Remove the C extension so that we can test the PyTracer