Add support for evaluate kwarg in relational classes

(Plus some pep8 fixes for sympy/core/tests/test_relational.py)
Update finite_diff.py

Fixed typos.
build_doc: fix a syntax error

This was introduced in #7787.
Merge pull request #7795 from certik/travis_fix

build_doc: fix a syntax error
Merge pull request #7778 from skirpichev/evaluate-kwarg-for-rel

Add support for evaluate kwarg in relational classes
Merge pull request #7794 from naslundx/patch-1

Update finite_diff.py
Merge branch 'master' into I-comp
Fix test_partial_simp