The constructor for `functools.partial()`

detects nesting and automatically flattens
itself to a more efficient form. For example:

```
from functools import partial
def f(*, a: int, b: int, c: int) -> None:
print(f"Args are {a}-{b}-{c}")
g = partial(partial(partial(f, a=1), b=2), c=3)
# Three function calls are flattened into one; free efficiency.
print(g)
# Bare function can be called as 3 arguments were bound previously.
g()
```

This returns:

```
functools.partial(<function f at 0x7f4fd16c11f0>, a=1, b=2, c=3)
Args are 1-2-3
```

## References

- Raymond Hettinger's tweet.