deps: backport dataclasses for Python 3.6

DifferentialOrange · DifferentialOrange · commit 976414e92354 · 2022-10-31T11:53:15.000+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 #232
diff --git a/requirements.txt b/requirements.txt
@@ -1,3 +1,4 @@
 msgpack>=1.0.4
 pandas
 pytz
+dataclasses; python_version <= '3.6'
