fix: correct open encoding

Using the wrong encoding creates issues on Windows

setup.py

@@ -1,7 +1,7 @@
 from setuptools import setup
 
 def readme():
-    with open('README.md', encoding="utf8") as f:
+    with open('README.md', encoding="utf-8") as f:
         return f.read()
 
 setup(name='diffeqpy',