fix arguments to pandas pd.merge

[Dr-Irv]

pylance 2022.2.0

The following code:

import pandas as pd

df1 = pd.DataFrame([["a", 1], ["b", 2]], columns=["let", "num"]).set_index("let")

s2 = df1["num"]

res = pd.merge(s2, df1, left_index=True, right_index=True)

print(res)

reports

Argument of type "Series[S1@__getitem__]" cannot be assigned to parameter "left" of type "DataFrame" in function "merge"
  "Series[S1@__getitem__]" is incompatible with "DataFrame"

This PR fixes that issue. Note it was previously fixed in #61 and then broken in #129 .