Improve Doc.get return value type (#80)

* Improve Doc.get return value type

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

python/pycrdt/_doc.py

@@ -1,13 +1,15 @@
 from __future__ import annotations
 
-from typing import Callable, Type, cast
+from typing import Callable, Type, TypeVar, cast
 
 from ._base import BaseDoc, BaseType, base_types
 from ._pycrdt import Doc as _Doc
 from ._pycrdt import SubdocsEvent, TransactionEvent
 from ._pycrdt import Transaction as _Transaction
 from ._transaction import ReadTransaction, Transaction
 
+T_BaseType = TypeVar("T_BaseType", bound=BaseType)
+
 
 class Doc(BaseDoc):
     def __init__(
@@ -73,7 +75,7 @@ def __getitem__(self, key: str) -> BaseType:
     def __iter__(self):
         return iter(self.keys())
 
-    def get(self, key: str, *, type: Type[BaseType]) -> BaseType:
+    def get(self, key: str, *, type: Type[T_BaseType]) -> T_BaseType:
         value = type()
         self[key] = value
         return value