Fix: add float to allowed cell values

[robertmetcalf]

Add float as a Cell.value.

[MartinThoma]

Thank you!

I'm currently abroad and without a laptop. I'm also sick - I'm still hoping I'm good enough to fly back🤞

What I want to say: it might take a couple of days until I release to pypi and I might completely forget it. If it's not done until Saturday, please send me a reminder 😅