Revert "Merge branch 'symengine' into 'master'"

.gitlab-ci.yml

.test-conda-env-py3-requirements.txt

.test-conda-env-py3.yml

doc/misc.rst

requirements.txt

sumpy/assignment_collection.py

sumpy/codegen.py

sumpy/cse.py

sumpy/e2e.py

sumpy/e2p.py

sumpy/expansion/__init__.py

sumpy/expansion/local.py

sumpy/expansion/multipole.py

sumpy/kernel.py

sumpy/p2e.py

sumpy/p2p.py

sumpy/qbx.py

sumpy/symbolic.py

sumpy/tools.py

sumpy/version.py