Fix encoding issue in setup.py for Simpay API wrapper

The commit fixes an issue where the README.md file was not being read correctly due to encoding. The change adds the 'utf-8' encoding parameter to the open() function call.

Author: DarkGL

setup.py

@@ -6,7 +6,7 @@
     name='simpay-api',
     version='3.0',
     description='Python wrapper for Simpay API',
-    long_description=open('README.md').read(),
+    long_description=open('README.md', encoding='utf-8').read(),
     long_description_content_type="text/markdown",
     author='Rafał Więcek',
     author_email="kontakt@simpay.pl",