find singularities for any expression

If `poles` is True and the function is not rational, an
error will be raised; if False, all values of `sym` that
set any denominator to zero are returned.

If `strict` is False (default) then removable
singularities are not reported:

1/(1/x - 1) is not defined for x in (0, 1) but only 1 is returned
with the default setting for `strict`.