[bugfix] round datetimes to the nearest full microsecond

[xs]

After we convert the microseconds string to a float and multiply by 1e6, it's possible that we lose a microsecond when casting to an integer again:

Example:

In [1]: usecs = float('0.524226') * 1e6

In [2]: usecs
Out[2]: 524225.99999999994

In [3]: int(usecs)
Out[3]: 524225

Test code:

old_errors = 0
new_errors = 0

for expected in range(1000000):
    usecs_string = '0.' + str(expected).zfill(6)

    usecs = float(usecs_string) * 1e6

    old_actual = int(usecs)
    new_actual = int(round(usecs))

    if old_actual != expected:
        old_errors += 1

    if new_actual != expected:
        new_errors += 1


print 'old_errors: ' + str(old_errors)
print 'new_errors: ' + str(new_errors)

Test output:

old_errors: 11549
new_errors: 0

[methane]

Please check PyMySQL's code before trying fix by yourself.
Otherwise, many different implementations are born. I can't maintain them.