Change testing config to use pytest instead of nosetests

Closes #5

setup.py

@@ -29,4 +29,5 @@ setup(
             'res2sqlite = galvani.res2sqlite:main',
     ]},
     install_requires=['numpy'],
+    tests_require=['pytest'],
 )

tox.ini

@@ -1,5 +1,5 @@
 [tox]
 envlist = py27,py35,py37
 [testenv]
-deps=nose
-commands=nosetests
+deps=pytest
+commands=pytest