compute_weak_subjectivity_period should use integer division (//)

[ericsson49]

Perhaps because of a typo, compute_weak_subjectivity_period uses non-integer division '/':

weak_subjectivity_period += SAFETY_DECAY * CHURN_LIMIT_QUOTIENT / (2 * 100)

This results in an exception raised:

>>> compute_weak_subjectivity_period(BeaconState())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/mnt/c/Users/ericsson/IdeaProjects/eth2.0-specs/tests/core/pyspec/eth2spec/phase1/spec.py", line 2072, in compute_weak_subjectivity_period
    weak_subjectivity_period += SAFETY_DECAY * validator_count / (2 * 100 * MIN_PER_EPOCH_CHURN_LIMIT)
  File "/mnt/c/Users/ericsson/IdeaProjects/eth2.0-specs/venv/lib/python3.8/site-packages/remerkleable/basic.py", line 116, in __truediv__
    raise OperationNotSupported(f"non-integer division '{self} / {other}' "
remerkleable.basic.OperationNotSupported: non-integer division '0 / 800' is not valid for uint64 left hand type

[djrtwo]

thanks!