Fix FloatConvertor regex (#1973)

* Bugfix of regex at FloatConvertor (version 2)

For passing your checks of #1942
A correct statement is: regex = r"[0-9]+(\.[0-9]+)?"
Reference: https://www.flake8rules.com/rules/W605.html

I have no problem to corrected without 'r' prefix directly at /site-packages/starlette/convertors.py of my computer.
Having submitted last pull-request, I realized to add a 'r' prefix to pass your tests.

* Add test

Co-authored-by: Ching <thus.kindly@gmail.com>

starlette/convertors.py

@@ -51,7 +51,7 @@ def to_string(self, value: int) -> str:
 
 
 class FloatConvertor(Convertor):
-    regex = "[0-9]+(.[0-9]+)?"
+    regex = r"[0-9]+(\.[0-9]+)?"
 
     def convert(self, value: str) -> float:
         return float(value)

tests/test_convertors.py

@@ -54,3 +54,17 @@ def test_datetime_convertor(test_client_factory, app: Router):
         app.url_path_for("datetime-convertor", param=datetime(1996, 1, 22, 23, 0, 0))
         == "/datetime/1996-01-22T23:00:00"
     )
+
+
+@pytest.mark.parametrize("param, status_code", [("1.0", 200), ("1-0", 404)])
+def test_default_float_convertor(test_client_factory, param: str, status_code: int):
+    def float_convertor(request):
+        param = request.path_params["param"]
+        assert isinstance(param, float)
+        return JSONResponse({"float": param})
+
+    app = Router(routes=[Route("/{param:float}", endpoint=float_convertor)])
+
+    client = test_client_factory(app)
+    response = client.get(f"/{param}")
+    assert response.status_code == status_code