Tests fail due to outsized integer test values

[dataflake]

The Python maintainers recently fixed python/cpython#95778, which puts a size limit on values converted between int and str. The test code is littered with integer test values that exceed this new limit, so tests fail like this:

Failed example:
[   15s]     f.fromUnicode(str(2**31234) + '.' + str(2**256))
[   15s]     # doctest: +ELLIPSIS
[   15s] Exception raised:
[   15s]     Traceback (most recent call last):
[   15s]       File "/usr/lib64/python3.8/doctest.py", line 1336, in __run
[   15s]         exec(compile(example.source, filename, "single",
[   15s]       File "<doctest zope.schema._field.Float[6]>", line 1, in <module>
[   15s]         f.fromUnicode(str(2**31234) + '.' + str(2**256))
[   15s]     ValueError: Exceeds the limit (4300) for integer string conversion