Set python_requires='>=3.6' - #936

From your changelog seems like in the latest version you dropped Python 2 support but it is incomplete. You need to set python_requires in your setup.py that will prevent users with Python 2.7 from downloading a sdist version they cannot build.

Note: supporting python_requires requires setuptools>=24.2.0 and pip>=9.0.0 to benefit from it.
Details here: https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

Here is the current issue if you try to install latest version of fastavro on Python 2:

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "c:\users\compile.rdv\appdata\local\temp\pip-install-_ifmxj\PyMySQL\setup.py", line 6, in <module>
        with open("./README.rst", encoding="utf-8") as f:
    TypeError: 'encoding' is an invalid keyword argument for this function

Also, the 1.0.0 version could be yanked.