Forbid `datetime.now` and `timezone.now` in default

[sobolevn]

People use timezone.now (sometimes even timezone.now()) as a default value. But, there are auto_now_add=True and auto_now=True for DateTimeField.

Sub-checks:

• timezone.now() is evaluated once, use timezone.now
• Don't use datetime.now, it does not have a proper timezone support

[kalekseev]

timezone.now() is evaluated once, use timezone.now

this case is checked by fields.W161 https://docs.djangoproject.com/en/3.1/ref/checks/#model-fields

But, there are auto_now_add=True and auto_now=True for DateTimeField

auto keywords and default=timezone.now yield different results, so I'm not sure that we should have opinion about that.

Don't use datetime.now, it does not have a proper timezone support

that one looks like a good candidate for check