Fix an issue when tox p fails because mypy is forbidden.

See details here: https://stackoverflow.com/questions/47642747/tox-warningtest-command-found-but-not-installed-in-testenv

src/flask_sqlalchemy/extension.py

@@ -952,9 +952,7 @@ def _set_rel_query(self, kwargs: dict[str, t.Any]) -> None:
 
             backref[1].setdefault("query_class", self.Query)
 
-    def relationship(
-        self, *args: t.Any, **kwargs: t.Any
-    ) -> sa_orm.Relationship[t.Any]:
+    def relationship(self, *args: t.Any, **kwargs: t.Any) -> sa_orm.Relationship[t.Any]:
         """A :func:`sqlalchemy.orm.relationship` that applies this extension's
         :attr:`Query` class for dynamic relationships and backrefs.
 
@@ -976,9 +974,7 @@ def dynamic_loader(
         self._set_rel_query(kwargs)
         return sa_orm.dynamic_loader(argument, **kwargs)
 
-    def _relation(
-        self, *args: t.Any, **kwargs: t.Any
-    ) -> sa_orm.Relationship[t.Any]:
+    def _relation(self, *args: t.Any, **kwargs: t.Any) -> sa_orm.Relationship[t.Any]:
         """A :func:`sqlalchemy.orm.relationship` that applies this extension's
         :attr:`Query` class for dynamic relationships and backrefs.
 

tox.ini

@@ -20,11 +20,13 @@ deps =
 commands = pytest -v --tb=short --basetemp={envtmpdir} {posargs}
 
 [testenv:style]
+allowlist_externals = mypy
 deps = pre-commit
 skip_install = true
 commands = pre-commit run --all-files
 
 [testenv:typing]
+allowlist_externals = mypy
 deps = -r requirements/mypy.txt
 commands =
     mypy --python-version 3.8