Improve typing in the module

Fixes pallets-eco#65

- make it pass mypy tests (i.e., improve type hinting)
- add mypy test to CI
- add py.typed to setup so that other packages recognize it's typed
- enable async view function decoration (via ensure_sync() as noted on
  https://flask.palletsprojects.com/en/latest/async-await/#extensions)

Signed-off-by: Marek Pikuła <[email protected]>

Author: MarekPikula

Diff for: .github/workflows/tests.yml

@@ -3,6 +3,26 @@ name: Tests
 on: [push, pull_request]
 
 jobs:
+  mypy:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Set up Python 3.7
+      uses: actions/setup-python@v1
+      with:
+        python-version: "3.7"
+    - name: Install dependencies
+      if: steps.cache-pip.outputs.cache-hit != 'true'
+      run: |
+        python -m pip install --upgrade pip
+        pip install -r requirements/test.pip
+    - name: Run mypy satic check
+      run: |
+        python3 -m mypy flask_pydantic/
+
   build:
     runs-on: ${{ matrix.os }}
     strategy:

Diff for: flask_pydantic/converters.py

@@ -1,12 +1,12 @@
-from typing import Type
+from typing import Dict, List, Type, Union
 
 from pydantic import BaseModel
-from werkzeug.datastructures import ImmutableMultiDict
+from werkzeug.datastructures import MultiDict
 
 
 def convert_query_params(
-    query_params: ImmutableMultiDict, model: Type[BaseModel]
-) -> dict:
+    query_params: "MultiDict[str, str]", model: Type[BaseModel]
+) -> Dict[str, Union[str, List[str]]]:
     """
     group query parameters into lists if model defines them