fix(utils/text): use proper type for format_lazy return type

[sshishov]

Use _StrPromise instead of _StrOrPromise for format_lazy return type.

Closes #2840

[sobolevn]

But, it can return str

In [1]: from django.utils.text import format_lazy

In [2]: format_lazy('a{0}', 'b')
Out[2]: 'ab'

[sshishov]

HI @sobolevn , it seems it just does not have the safety-net. But yeah if it can accept str and return str then we have to type it properly. We can close my MR with the statement that Working As Expected.

It is just useless to use format_lazy for non-lazy objects, you can use f-string or str.format instead.

To manage mypy-error in our code we will add the internal assert then, which should fail on dev runtime for us. It is also suitable 👍

-- Sergei

[sobolevn]

Maybe we can use overloads?