pd.DataFrame.astype should allow dict as argument

[maxnoe]

Currently pd.DataFrame.astype only allows for one type.

I would like it to support a dict mapping column names to dtypes.
Like for example the dtype argument of pd.read_json.

df = pd.DataFrame({'a': [1, 2, 3], 'b': [1, 2, 3]})
df = df.astype({'a': 'int16', 'b': 'float32'})

[jreback]

dupe of #7271

PR is #12086

welcome to have you pick it up.