Move from nose to pytest for unit tests

doc/development.rst

doc/install.rst

requirements.txt

tests/test_crossmap.py

tests/test_describe.py

tests/test_grammar.py

tests/test_mapping.py

tests/test_mutator.py

tests/test_parsers_genbank.py

tests/test_scheduler.py

tests/test_services_json.py

tests/test_services_soap.py

tests/test_variantchecker.py

tests/test_website.py