Fix setup.py test invocation (#263)

.gitignore

@@ -5,6 +5,7 @@
 *.egg
 *.egg-info
 .coverage
+.eggs
 .DS_Store
 __pycache__
 build

setup.cfg

@@ -0,0 +1,2 @@
+[aliases]
+test=pytest

setup.py

@@ -77,6 +77,7 @@ setup(
     packages = find_packages("src"),
     package_dir = {"": "src"},
     ext_modules = [tokenizer] if use_extension else [],
+    setup_requires = ["pytest-runner"] if "test" in sys.argv or "pytest" in sys.argv else [],
     tests_require = ["pytest"],
     version = __version__,
     python_requires = ">= 3.5",