Prevent package from installing on Python < 3.6 (#195)

Author: dolfinus

fluent.runtime/setup.py

@@ -28,6 +28,7 @@
       packages=['fluent.runtime'],
       package_data={'fluent.runtime': ['py.typed']},
       # These should also be duplicated in tox.ini and /.github/workflows/fluent.runtime.yml
+      python_requires='>=3.6',
       install_requires=[
           'fluent.syntax>=0.17,<0.20',
           'attrs',