Fix Series#get and DataFrame#get

[skatsuta]

• Closes pd.Series.get causes typing error in second argument (default value) #808
• Tests added: Please use assert_type() to assert the type of any return value

Fix Series#get and DataFrame#get to resolve #808.

[skatsuta]

@Dr-Irv Thanks for your suggestions.

Added the following changes:

• Used _typing.T instead of locally defined type vars
• Removed = ... from the overloads for cases where the default parameter is specified
• Split Series#get(..., default: S1) -> S1 and Series#get(..., default: T) -> S1 | T
• Added test cases for when the default parameter is None and no key is found

[Dr-Irv]

thanks @skatsuta