fix series.astype dtype argument for pandas

[Dr-Irv]

With pylance 2021.5.3, the following code

import pandas as pd

df = pd.DataFrame({"x": [1.0, 2.0, 3.0], "y": [4.0, 5, 6]})

sx: pd.Series = df["x"]

ss = sx.astype(str)

print(ss)

reports on the astype line:

Argument of type "Type[str]" cannot be assigned to parameter "dtype" of type "Dtype@__getitem__ | str" in function "astype"
  Type "Type[str]" cannot be assigned to type "Dtype@__getitem__ | str"
    Type "Type[str]" cannot be assigned to type "Dtype@__getitem__"
    "Type[type]" is incompatible with "_str"

This PR fixes that issue.

[jakebailey]

Thank you!