Python: add support for int and float payloads

[jfaust]

As referenced in #89

In addition to the basic unit tests, I've confirmed that nsys and Nsight systems see the payloads properly via a simple script:

import nvtx
import time

for i in range(0, 100):
    with nvtx.annotate('loop', payload=i):
        nvtx.mark('intvalue', payload=i)
        nvtx.mark('floatvalue', payload=float(i))
        time.sleep(0.01)

[shwina]

Thank you!