maint(precommit): Apply isort (#3195)

* Apply isort

* Tweak isort config

* Add env.py as a known_first_party

* Add one missing known first party

* Make config compat with older isort versions

* Add another comment

* Revert pyproject setting

pyproject.toml

@@ -15,6 +15,12 @@ ignore = [
     "noxfile.py",
 ]
 
+[tool.isort]
+# Needs the compiled .so modules and env.py from tests
+known_first_party = "env,pybind11_cross_module_tests,pybind11_tests,"
+# For black compatibility
+profile = "black"
+
 [tool.mypy]
 files = "pybind11"
 python_version = "2.7"