Add a note to the dev docs on mypy (#10485)

extra/DEVELOPER_DOCS/Code Conventions.md

@@ -137,7 +137,7 @@ If any of the TODOs you've added are important and should be fixed soon, you sho
 
 ## Type hints
 
-We use Python type hints across the `.py` files wherever possible. This makes it easy to understand what a function expects and returns, and modern editors will be able to show this information to you when you call an annotated function. Type hints are not currently used in the `.pyx` (Cython) code, except for definitions of registered functions and component factories, where they're used for config validation.
+We use Python type hints across the `.py` files wherever possible. This makes it easy to understand what a function expects and returns, and modern editors will be able to show this information to you when you call an annotated function. Type hints are not currently used in the `.pyx` (Cython) code, except for definitions of registered functions and component factories, where they're used for config validation. Ideally when developing, run `mypy spacy` on the code base to inspect any issues.
 
 If possible, you should always use the more descriptive type hints like `List[str]` or even `List[Any]` instead of only `list`. We also annotate arguments and return types of `Callable` – although, you can simplify this if the type otherwise gets too verbose (e.g. functions that return factories to create callbacks). Remember that `Callable` takes two values: a **list** of the argument type(s) in order, and the return values.
 
@@ -155,6 +155,13 @@ def create_callback(some_arg: bool) -> Callable[[str, int], List[str]]:
     return callback
 ```
 
+For typing variables, we prefer the explicit format.
+
+```diff
+- var = value    # type: Type
++ var: Type = value
+```
+
 For model architectures, Thinc also provides a collection of [custom types](https://thinc.ai/docs/api-types), including more specific types for arrays and model inputs/outputs. Even outside of static type checking, using these types will make the code a lot easier to read and follow, since it's always clear what array types are expected (and what might go wrong if the output is different from the expected type).
 
 ```python