docs: update Dev. Status from "Alpha" to "Production/Stable"

... as the project is already widely used and it seems stable enough already.

setup.py

@@ -11,7 +11,7 @@
       packages=['mistletoe'],
       entry_points={'console_scripts': ['mistletoe = mistletoe.__main__:main']},
       classifiers=[
-          'Development Status :: 3 - Alpha',
+          'Development Status :: 5 - Production/Stable',
           'Intended Audience :: Developers',
           'License :: OSI Approved :: MIT License',
           'Programming Language :: Python :: 3',