Error in mypy plugin reading config from pyproject.toml with mypy >= 0.920

[paul-sud]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.9.0a2
            pydantic compiled: True
                 install path: /Users/paul/.cache/pre-commit/repo8u79aah5/py_env-python3.9/lib/python3.9/site-packages/pydantic
               python version: 3.9.6 (default, Aug 18 2021, 18:03:48)  [Clang 11.0.3 (clang-1103.0.32.62)]
                     platform: macOS-10.16-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions']

I'm testing out the new pyproject.toml config support in alpha. I tried to set the config in my pyproject.toml exactly like in the docs: https://pydantic-docs.helpmanual.io/mypy_plugin/#configuring-the-plugin . However, this doesn't seem to work with mypy 0.930, when I run it I get the folllowing traceback:

Error constructing plugin instance of PydanticPlugin

Traceback (most recent call last):
  File "/Users/paul/.cache/pre-commit/repo8u79aah5/py_env-python3.9/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/Users/paul/.cache/pre-commit/repo8u79aah5/py_env-python3.9/lib/python3.9/site-packages/mypy/__main__.py", line 12, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "mypy/main.py", line 96, in main
  File "mypy/main.py", line 173, in run_build
  File "mypy/build.py", line 180, in build
  File "mypy/build.py", line 231, in _build
  File "mypy/build.py", line 478, in load_plugins
  File "mypy/build.py", line 456, in load_plugins_from_config
  File "pydantic/mypy.py", line 91, in pydantic.mypy.PydanticPlugin.__init__
  File "pydantic/mypy.py", line 129, in pydantic.mypy.PydanticPluginConfig.__init__
  File "pydantic/mypy.py", line 130, in pydantic.mypy.PydanticPluginConfig.__init__
  File "/Users/paul/.cache/pre-commit/repo8u79aah5/py_env-python3.9/lib/python3.9/site-packages/tomli/_parser.py", line 56, in load
    s = __fp.read().decode()
AttributeError: 'str' object has no attribute 'decode'

The issue is here: https://github.com/samuelcolvin/pydantic/blob/ef4678999f94625819ebad61b44ea264479aeb0a/pydantic/mypy.py#L129

Newer mypy versions (>=0.920) use tomli for parsing, and tomli's version of load expects the file handle in binary mode explaining the error.

Related:
#2895
#2908

[samuelcolvin]

Thanks for reporting, I've fixed this in #3594 (build currently failing due to issues with pypi).