Improve Doc.get return value type

[davidbrochart]

With this change, mypy knows the return type of Doc.get("foo", type=MyType) is MyType, while it was previously BaseType. The type passed to type must still be a BaseType.
This can be seen by running mypy test.py:

# test.py
from pycrdt import Doc, Text

doc = Doc()
text = doc.get("text", type=Text)
print(reveal_type(text))
# note: Revealed type is "pycrdt._text.Text"
doc.get("foo", type=int)
# error: Value of type variable "T_BaseType" of "get" of "Doc" cannot be "int"  [type-var]