setup.py: constrain Python version

Installation should be constraint to supported Python versions, using `python_requires`,
refer to [1] for details.

[1] https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

setup.py

@@ -16,6 +16,7 @@
     license="BSD",
     install_requires=["pyvcd>=0.1.4", "bitarray"],
     packages=find_packages(),
+    python_requires=">=3.6",
     project_urls={
         #"Documentation": "https://glasgow.readthedocs.io/",
         "Source Code": "https://github.com/m-labs/nmigen",