MergeHow does not support "cross" joins

[alexcolemandata]

Describe the bug
Performing a cross join with two dataframes with df.merge(..., how="cross", ...) creates a typing error `Type "Literal['cross']" cannot be assigned to type "MergeHow"

To Reproduce

Minimal example

import pandas as pd

df1 = pd.DataFrame(data={"a": [1, 2, 3]})
df2 = pd.DataFrame(data={"b": ["x", "y", "z"]})

df3 = df1.merge(df2, how="cross")

Pyright

Found 1 source file
pyright 1.1.270
[...]
[...]/cross_merge.py:6:26 - error: Argument of type "Literal['cross']" cannot be assigned to parameter "how" of type "MergeHow" in function "merge"
    Type "Literal['cross']" cannot be assigned to type "MergeHow"
      "Literal['cross']" cannot be assigned to type "Literal['left']"
      "Literal['cross']" cannot be assigned to type "Literal['right']"
      "Literal['cross']" cannot be assigned to type "Literal['outer']"
      "Literal['cross']" cannot be assigned to type "Literal['inner']" 

Please complete the following information:

• OS: MacOS
• OS Version: 12.5.1
• python version: 3.10.5
• version of type checker: pyright 1.1.270
• version of installed pandas-stubs: 1.4.4.220906

Additional context
L231: _typing.pyi

pandas DataFrame.merge docs - describes how="cross" option.

[Dr-Irv]

Closing via #355