Remove upper limit on pyparsing

See discussion in : pypa#480
`pyparsing` is backward compatible for now (synonyms for camel cased
attributes).
Fix unit test to accecpt either pyparsing 2 or 3 exception message.

setup.py

@@ -48,7 +48,7 @@
     author=about["__author__"],
     author_email=about["__email__"],
     python_requires=">=3.6",
-    install_requires=["pyparsing>=2.0.2,<3"],  # Needed to avoid issue #91
+    install_requires=["pyparsing>=2.0.2"],  # Needed to avoid issue #91
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",

tests/test_requirements.py

@@ -192,4 +192,6 @@ def test_sys_platform_linux_in(self):
     def test_parseexception_error_msg(self):
         with pytest.raises(InvalidRequirement) as e:
             Requirement("toto 42")
-        assert "Expected stringEnd" in str(e.value)
+        assert "Expected stringEnd" in str(e.value) or (
+            "Expected string_end" in str(e.value)  # pyparsing>=3.0.0
+        )