use jaconv instead of jcconv for japanese chars

jaconv is available for more Python-versions and seems to be more
professional. Apart from that I added jaconv to the test-requirements
but not the requirements. (If the katakana-stuff really works we can
later add it as a real dependency)

test/test_magicencode.py

@@ -95,12 +95,12 @@ class TestMagicEncode:
 
 
 try:
-    import jcconv
+    import jaconv
 except ImportError:
-    jcconv = None
+    jaconv = None
 
 
-@pytest.mark.skipif(not jcconv, reason="jcconv not installed")
+@pytest.mark.skipif(not jaconv, reason="jaconv not installed")
 class TestKatakana:
     @given(st.text())
     @example("カタカナ")