chore(jalai.py): cleanup the code

[5j9]

• Remove unneeded global statements.
• Remove unneeded intermediate variable declarations.
• Do not call int on floor division result. This is not needed in Python 3. Floor division of ints always returns an integer in Python 3.
• Use augmented arithmetic assignments (%=, -=, etc.) when possible.
• Add a test for conversion on leap gregorian years. I noticed that there are no tests for this particular situation.

[5j9]

Some changes are perhaps a little subjective. Feel free to reject if the old code looks more readable to you.

[slashmili]

Thanks!