Pin version of ANTLR4 package

[markborkum]

Description

Use the most recent minor version of ANTLR4 prior to the release of version 4.8.

See https://pypi.org/project/antlr4-python3-runtime/#history for details.

Issues Resolved

Fixes #42

Check List

• All tests pass.
• New functionality includes testing.
• New functionality has been documented in the README if applicable

[dmlb2000]

@markborkum I think you could do something like <4.8 and that might be better

[markborkum]

@dmlb2000 ~=4.7 would be safer (since the underlying issue is breaking changes introduced by minor versions, this would also guard against out-of-date package managers that serve 4.6 or lower). Regardless, there are not going to be any new releases below 4.8, so let's stick with the explicit version, please.