Pendulum periods here, there and in a lift

You will write three functions for a simple pendulum.

The idea. For small swings, $T = 2\pi\sqrt{L/g}$. To find the length for a chosen period, square it: $L = \dfrac{gT^2}{4\pi^2}$. In a lift with acceleration $a$, upwards positive, the bob feels $g + a$.

Your tasks.

1. In period, return $T$ in s for a length $L$ in m and gravity $g$ in m/s².
2. In length_for, return the length in m that gives the period $T$.
3. In period_in_lift, return the period in a lift. If $g + a$ is zero or less, return None, because the bob does not swing.