Summary:
Python isdigit() returns true for unicode characters such as "²", which fail
when used as an argument to int().
Instead, be explicit about only considering characters 0-9 to be digits.
Test Plan: Added a test case which produces an error without this change.
Reviewers: alexmojaki
Reviewed By: alexmojaki
Differential Revision: https://phab.getgrist.com/D3027
