"none is not an allowed value" when using Union with Any

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

It seems that None is allowed when a field's type is Any (as is expected, stated in the docs), but None is not allowed when a field's type is Union[SomeType, Any].

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

             pydantic version: 1.8.2
            pydantic compiled: True
                 install path: C:\Users\TobyHarradine\PycharmProjects\orchestration\.venv\Lib\site-packages\pydantic
               python version: 3.7.9 (tags/v3.7.9:13c94747c7, Aug 17 2020, 18:58:18) [MSC v.1900 64 bit (AMD64)]
                     platform: Windows-10-10.0.19041-SP0
     optional deps. installed: ['typing-extensions']

Code which reproduces the issue:

>>> import pydantic
>>> class M(pydantic.BaseModel):
...     a: Any
...
>>> M(a=1)
M(a=1)
>>> M(a=None)
M(a=None)
>>> class M(pydantic.BaseModel):
...     a: Union[int, Any]
...
>>> M(a=1)
M(a=1)
>>> M(a="abcd")
M(a='abcd')
>>> M(a=None)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "pydantic\main.py", line 406, in pydantic.main.BaseModel.__init__
pydantic.error_wrappers.ValidationError: 1 validation error for M
a
  none is not an allowed value (type=type_error.none.not_allowed)

This may be somewhat related to #1624.

[mykhailoleskiv]

allow_none attribute for field a remains False. Possible fix may be in pydantic/fields.py:566:

if is_union_origin(origin):
    types_ = []
    for type_ in get_args(self.type_):
        if type_ is NoneType or type_ is Any or type_ is object:
            if self.required is Undefined:
                self.required = False
            self.allow_none = True
            continue
        types_.append(type_)

@mykhailoleskiv That almost works, except it doesn't add Any or object to types_ (which eventually becomes ModelField.sub_fields). So M(a="abcd") or something like that no longer works. I'll open a PR