Do not use mixed units in timeit output

Timeit reports mean and std dev in different units in some cases, e.g. the mean is in a different order of magnitude from the std.

In [1]: from time import sleep

In [2]: %timeit sleep(0.001)
1.08 ms ± 11.4 μs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

This is pretty hard to parse and violates best practices of reporting value and uncertainty in the same units.

The preferred output would be:

In [1]: from time import sleep

In [2]: %timeit sleep(0.001)
1.080 ± 0.011 ms per loop (mean ± std. dev. of 7 runs, 1,000 loops each)