s=', '.join(['{0} to {1}'.format(x[0], x[1]) for x in Availability])
print('You have availability ' + s)

You have availability 8:00 to 8:45, 9:15 to 9:30, 10:00 to 10:15.

s=', '.join(['{0} to {1}'.format(x[0], x[1]) if x!=a[-1] else 'and {0} to {1}'.format(x[0], x[1]) for x in a])
print('You have availability ' + s)

You have availability 8:00 to 8:45, 9:15 to 9:30, and 10:00 to 10:15