adjust isort config (#4290)

Fixes #4288.

- Default imports to THIRD_PARTY, so in effect we merge the FIRST_PARTY and THIRD_PARTY stubs. This means import order is no longer affected by whether typing_extensions is installed locally.
- Treat typing_extensions, _typeshed and some others as standard library modules.

Note that isort master is very different from the latest release; we'll have to do something
different if and when the next isort release comes out.

pyproject.toml

@@ -9,3 +9,5 @@ force_grid_wrap = 0
 use_parentheses = true
 combine_as_imports = true
 line_length = 130
+default_section = "THIRDPARTY"
+known_standard_library = ["typing_extensions", "_typeshed", "_compression", "_markupbase", "opcode"]