Add missing return type annotation to json_extension.dump() (#195)

The `dump()` function wrapper was missing a return type annotation,
causing type checking ambiguity.

## Changes
- Added `-> None` return type annotation to `dump()` in
`gridappsd/json_extension.py`

The function wraps `json.dump()`, which writes to a file object and
returns `None`. The annotation now matches the standard library's
signature.

```python
def dump(
    data: Any,
    fo: TextIO,
    *,
    skipkeys=False,
    ensure_ascii=True,
    check_circular=True,
    allow_nan=True,
    indent=None,
    separators=None,
    default=None,
    sort_keys=False,
    **kw,
) -> None:  # Added
    _json.dump(...)
```

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Author: craig8

gridappsd-python-lib/gridappsd/json_extension.py

@@ -46,7 +46,7 @@ def dump(
     default=None,
     sort_keys=False,
     **kw,
-):
+) -> None:
     _json.dump(
         data,
         fo,