Currying support for generic attrs classes

# Bug report

I tried out returns, because I was looking for typesafe curry implementation, but I hit a problem with my Generic / Protocol implementing attrs classes.

## What's wrong
```python
from typing import TypeVar, Generic

from attr import frozen
from returns.curry import curry

T = TypeVar("T")

@curry
@frozen(kw_only=True)
class Pair(Generic[T]):
_a: T
_b: T

p = Pair(b=1)(a=2)
print(p)

```
```
TypeError: __init__() missing 1 required keyword-only argument: 'a'
```

## How is that should be

The above code should simply print

`Pair(_a=2, _b=1)`

This seems to be due to https://github.com/python-attrs/attrs/issues/374

A fix I hacked together could be something like this.

```python
def curry(function: Callable[..., _ReturnType]) -> Callable[..., _ReturnType]:
import inspect
if inspect.isclass(function):
init_signature = Signature.from_callable(function.__init__)
init_params_without_self = tuple(init_signature.parameters.values())[1:]
argspec = init_signature.replace(parameters=init_params_without_self).bind_partial()
else:
argspec = Signature.from_callable(function).bind_partial()

def decorator(*args, **kwargs):
return _eager_curry(function, argspec, args, kwargs)
return wraps(function)(decorator)
```

If you are interested in support this admittedly special use case, I could submit a PR.

## System information

- `python` version: 3.7
- `returns` version: 0.17.0
- `mypy` version: 0.910

Start at the returns.curry.curry entry point and trace how _eager_curry binds arguments for classes versus regular callables. Reproduce the Generic, frozen, keyword-only Pair example, then verify that supplying b first and a second produces Pair(_a=2, _b=1) without the missing-argument error.

Scritto dal modello di indicizzazione a partire dal testo della issue.