Unpin Python version and dont reread README.md in setup.py

Unpin Python version and use the long_description that was read earlier instead of rereading it.

setup.py

@@ -16,15 +16,15 @@ def get_version():
     name="sphinx-tabs",
     version=get_version(),
     description="Tabbed views for Sphinx",
-    long_description=open("README.md").read(),
+    long_description=long_description,
     long_description_content_type="text/markdown",
     author="djungelorm",
     author_email="[email protected]",
     packages=["sphinx_tabs"],
     include_package_data=True,
     url="https://github.com/executablebooks/sphinx-tabs",
     license="MIT",
-    python_requires="~=3.7",
+    python_requires=">=3.7",
     install_requires=["sphinx", "pygments", "docutils"],
     extras_require={
         "testing": [