Force include dataclasses in the binary.

Let's see if that fixes the issue with Python 3.6 in Centos build image.

Makefile

@@ -2,7 +2,7 @@
 all: dist/mnpw
 
 dist/mnpw: mnpw/*.py pyproject.toml *.rst LICENSE.txt
-	pyinstaller --noconfirm --onefile "--name=$$(basename $@)" mnpw/__init__.py
+	pyinstaller --noconfirm --onefile "--name=$$(basename $@)" --hidden-import dataclasses  mnpw/__init__.py
 
 .PHONY: clean
 clean: