pd.merge allows `Series` as first argument

[Dr-Irv]

Despite not being documented, the following works:

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)

pylance 2021.5.3 reports

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

Basically, a Series can be the first argument to the various pd.merge() functions.

PR to follow. Will also do a pandas PR to fix the docs