Class __init__ "Too few arguments" should be a better error message #9340

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?

Feature request

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

class Foo:
    def __init__(self, bar: int):
        pass


f = Foo()

• What is the actual behavior/output?

➜ python test.py
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    f = Foo()
TypeError: __init__() missing 1 required positional argument: 'bar'

~ via ⬢ 13.8.0 via 🐍 3.8.1 
➜ mypy --pretty test.py
test.py:6: error: Too few arguments for "Foo"
    f = Foo()
        ^
Found 1 error in 1 file (checked 1 source file)

• What is the behavior/output you expect?

mypy should say which arguments are missing, like Python does. This was fixed for functions in #505, but it doesn't seem to work for class init.

• What are the versions of mypy and Python you are using?

0.782, 3.8.1

• Do you see the same issue after installing mypy from Git master?

I have not tested this.

• What are the mypy flags you are using? (For example --strict-optional)

--pretty

• If mypy crashed with a traceback, please paste
  the full traceback below.

(You can freely edit this text, please remove all the lines
you believe are unnecessary.)