declare that tests require mecab-python3

Former-commit-id: 7b4ebd1805

setup.py

@@ -45,4 +45,10 @@ setup(
     packages=['wordfreq'],
     include_package_data=True,
     install_requires=dependencies,
+
+    # mecab-python3 is required for looking up Japanese word frequencies. In
+    # turn, it depends on libmecab-dev being installed on the system. It's not
+    # listed under 'install_requires' because wordfreq should be usable in
+    # other languages without it.
+    tests_require=['mecab-python3'],
 )