docs(internals): Add note of mypy in CI

Author: tony

CHANGES

@@ -35,6 +35,12 @@ $ pip install --user --upgrade --pre libtmux
 - `retry()`: Add deprecation warning. This will be removed in 0.13.x ({issue}`368`, {issue}`372`)
 - New function `retry_until()`: Polls a callback function for a set period of time until it returns `True` or times out. By default it will raise {exc}`libtmux.exc.WaitTimeout`, with `raises=False` it will return `False`. Thank you @categulario! ({issue}`368`, {issue}`372`)
 
+### Internals
+
+- {issue}`382` [mypy] support added:
+
+  - Basic mypy tests now pass
+
 ## libtmux 0.11.0 (2022-03-10)
 
 ### Compatibility