Improve typing

Fix pytest-dev#1057

changelog/1057.trivial

@@ -0,0 +1 @@
+The internals of pytest-xdist are now fully typed. The typing is not exposed yet.

pyproject.toml

@@ -137,19 +137,8 @@ lines-after-imports = 2
 [tool.mypy]
 mypy_path = ["src"]
 files = ["src", "testing"]
-# TODO: Enable this & fix errors.
-# check_untyped_defs = true
-disallow_any_generics = true
-ignore_missing_imports = true
-no_implicit_optional = true
-show_error_codes = true
-strict_equality = true
-warn_redundant_casts = true
-warn_return_any = true
+strict = true
 warn_unreachable = true
-warn_unused_configs = true
-# TODO: Enable this & fix errors.
-# no_implicit_reexport = true
 
 
 [tool.towncrier]