Place reserved_keywords.txt not in root.

Do not use data_files in setup.py but MANIFEST.in
Otherwise some enviroments throw errors when trying
to create the data file.

This was raised in: https://github.com/spulec/moto/pull/2885#discussion_r415150276

MANIFEST.in

@@ -3,5 +3,6 @@ include requirements.txt requirements-dev.txt tox.ini
 include moto/ec2/resources/instance_types.json
 include moto/ec2/resources/amis.json
 include moto/cognitoidp/resources/*.json
+include moto/dynamodb2/parsing/reserved_keywords.txt
 recursive-include moto/templates *
 recursive-include tests *

setup.py

@@ -101,5 +101,4 @@ setup(
     project_urls={
         "Documentation": "http://docs.getmoto.org/en/latest/",
     },
-    data_files=[('', ['moto/dynamodb2/parsing/reserved_keywords.txt'])],
 )