String dtype: disallow specifying the 'str' dtype with storage in [..] in string alias

[jorisvandenbossche]

The intention was for the new default "str" dtype to not include the storage in the string alias, and so to also not allow constructing it that way (this is discussed in the PDEP).

This is also implemented this way, as you can see when directly calling the extension dtype API:

# for "string" this is allowed
>>> pd.StringDtype.construct_from_string("string[pyarrow]")
string[pyarrow]

# but for "str" not
>>> pd.StringDtype.construct_from_string("str[pyarrow]")
...
TypeError: Cannot construct a 'StringDtype' from 'str[pyarrow]'

However, when specifying this as a dtype argument in eg constructors (going through pandas_dtype(...), which goes through the extension dtype registry), this "accidentally" kind of works, but gives an unexpected result:

# this raises as expected
>>> pd.Series(["a"], dtype="str[python]")
TypeError: data type 'str[python]' not understood

# but this gives a result
>>> pd.Series(["a"], dtype="str[pyarrow]")
>>>
0    a
dtype: string[pyarrow]
>>> type(pd.Series(["a"], dtype="str[pyarrow]").dtype)
pandas.ArrowDtype

I think it is confusing that it does work in case of the pyarrow storage, but then does give a different dtype than what you would typically expect.
So I would rather just disallow this case (which is what this PR does), although this is a small breaking change for people currently using dtype="str[pyarrow]" to get the ArrowDtype.

[rhshadrach]

I think it is confusing that it does work in case of the pyarrow storage, but then does give a different dtype than what you would typically expect. So I would rather just disallow this case (which is what this PR does), although this is a small breaking change for people currently using dtype="str[pyarrow]" to get the ArrowDtype.

This gives a definitively wrong result and so I think that puts it solidly in the bugfix camp, not a breaking change.

[WillAyd]

If we really cared we could convert that during the pickle read, although I dont think that would be a blocker. Generally using pickle to move from one environment to another is discouraged