Move AlgebraicNumber class to the core

fixes #8066
fixing error message in integrals
Merge pull request #8076 from skirpichev/8066-move-AlgebraicNumber

Move AlgebraicNumber class to the core
7322: nsimplify returns Rational instead of truncated 0 when rational=True
Merge pull request #8096 from smichr/solve

7322: nsimplify returns Rational instead of truncated 0
Backport mpmath's fix for py.test from https://github.com/fredrik-johansson/mpmath/pull/17
Rename test_numerically to verify_numerically.  The name
'test_numerically' was confusing to py.test and caused strange 'fixture' errors.
Make --slow option with py.test _only_ tests slow tests
Merge pull request #7931 from kaushik94/errorMessage

fixes #7704
Merge pull request #8099 from skirpichev/py.test-mpmath

Backport mpmath's fix for py.test
Merge pull request #8100 from pbrady/test_numerically

Rename test_numerically to verify_numerically for py.test compatibility
Merge pull request #8101 from pbrady/pytest_only_slow

Make --slow option for py.test consistent with bin/test usage
Ensure that theano are not run under py.test if theano is not installed
replace is with == comparison
reformat comments
additional bounded -> finite changes
add docstring to as_dummy()
sort when prining tree; unSKIP doctest
only print non-None keys
use two symbols