BUG: read_csv now errors with non-dict dtype and same-name columns

[fiendish]

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• I have confirmed this bug exists on the latest version of pandas.

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

with open("test.csv", "w") as cf:
    cf.write("A,B,B\n1,1,1")

pandas.read_csv("test.csv", engine="python", dtype=str)  # used to work, no longer works
pandas.read_csv("test.csv", engine="python", dtype="str")  # used to work, no longer works

Now produces

  File ".../pandas/io/parsers/python_parser.py", line 427, in _infer_columns
    and self.dtype.get(old_col) is not None
AttributeError: 'str' object has no attribute 'get'

Problem description

Documentation for read_csv says that dtype may be a "type name or dict", but the new parser behavior demands a dict in this scenario.

I believe the bug was introduced in 76792f1

Expected Output

A dataframe using str dtypes

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 2dd9e9b
python : 3.9.5.final.0
python-bits : 64
OS : Darwin
OS-release : 19.6.0
Version : Darwin Kernel Version 19.6.0: Mon Apr 12 20:57:45 PDT 2021; root:xnu-6153.141.28.1~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.3.0rc1
numpy : 1.20.3
pytz : 2021.1
dateutil : 2.8.1
pip : 21.0.1
setuptools : 56.2.0
Cython : None
pytest : 6.2.4
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : 2.8.6 (dt dec pq3 ext lo64)
jinja2 : 3.0.1
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : 3.0.7
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : 1.3.20
tables : None
tabulate : 0.8.7
xarray : None
xlrd : 1.2.0
xlwt : None
numba : None