Handle leap day birthdays by moving them one day earlier

[awendland]

This fix is unideal for the long term (since every four years these birthdays will be 1 day off) but I thought that it might be more useful in the short term than having the program exit.

[johnameen]

The reason leap years break his code is actually because it inserts the leap year birthday out of order-- in between all the 02/28 birthdays. If you move the 02/29 birthday to after all the 02/29 birthdays in the list, you won't have to incorrectly define someone's birthday as 02/28.

[awendland]

@johnameen I'm going to need some more assistance understanding the error case that you're talking about.

The error I'm addressing here (which I should have included in my hastily written PR description) is an exception being thrown by the datetime module when it's being supplied an invalid date (since February 29th doesn't occur in every year) through e.begin.

Here's an example of replicating the error in a Python REPL:

>>> from datetime import date
>>> date(2020, 2, 29)
datetime.date(2020, 2, 29)
>>> date(2021, 2, 29)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: day is out of range for month

This program encounters the error case because it creates a calendar event in the next year for birthdays that have already occurred in the given year (which happens to be a leap year, otherwise this error wouldn't occur).

[mobeigi]

@awendland Thanks for this! I slightly modified your commit hope you don't mind.