deps: backport dataclasses for Python 3.6

DifferentialOrange · DifferentialOrange · commit a1409b896b33 · 2022-10-13T17:33:44.000+03:00
Dataclasses would be a really convenient approach to provide box.error support. They are supported in Python since 3.7, but this package make it able to introduce their support in Python 3.6 which we still support. Part of #240
diff --git a/requirements.txt b/requirements.txt
@@ -1,3 +1,4 @@
 msgpack>=1.0.4
 pandas
 pytz
+dataclasses; python_version <= '3.6'
