Update pyparsing requirement from ~=2.0 to >=2,<4 (#296)

Do so only for Python 3 since pyparsing 3 dropped support for Python 2

Replaces #276

Author: ecederstrand

setup.py

@@ -52,7 +52,10 @@ def run_tests(self):
     packages=[
         'pyhocon',
     ],
-    install_requires=['pyparsing~=2.0'],
+    install_requires=[
+        'pyparsing~=2.0;python_version<"3.0"',
+        'pyparsing>=2,<4;python_version>="3.0"',
+    ],
     extras_require={
         'Duration': ['python-dateutil>=2.8.0']
     },