modify/correct Integral.is_number

isyms should start with the free symbols of the integrand, not all
symbols.

A 2nd non-heroic test for a case where a free symbol will not
affect evaluation was added to is_number and to evalf for integrals.
If the variable of integration does not appear in the free symbols
then only the difference in the limits is of significance, e.g.

Integral(x, (y, d, d + 1)) will introduce a linear factor of y
and the value of d is not significant. This integral is the same
as Integral(x, (y, 0, 1)).
add additional is_number test for Integral

[skip ci]
Merge pull request #7521 from smichr/n

modify/correct Integral.is_number
sympy.sets: Add is_proper_subset() and is_proper_superset() methods

Having these methods will not force one to make use of the < or >
operators for these operations only, since we already have is_subset()
and is_superset()  methods.
sympy.sets: Remove blank lines from doctest examples