Change tuple[HeaderType] to tuple[bytes] for Headers.get method (#642)

Author: tyzhnenko

blacksheep/headers.pyi

@@ -16,7 +16,7 @@ class Headers:
     def __init__(self, values: list[HeaderType] | None = None):
         self.values = values
 
-    def get(self, name: bytes) -> tuple[HeaderType]: ...
+    def get(self, name: bytes) -> tuple[bytes]: ...
     def get_tuples(self, name: bytes) -> list[HeaderType]: ...
     def get_first(self, key: bytes) -> bytes | None: ...
     def get_single(self, key: bytes) -> bytes: ...