Feature request: `sheet_names()` in python

[BKSalman]

Hi,

I wanted to run a script that calculates the average of values across multiple sheets, but the sheets are dynamic, so their names are not known beforehand, but the python API doesn't look like it exposes something to query the sheets names, and be able to iterate through them,

So having a function that look something like sheets_names() that returns a list is going to be very useful.

that's my feature request, and thank you for making this awesome project :)