Fixup dev environment

[joshuatz]

There are some barriers to getting a proper dev environment up and running for working on this project.

Most noticeably, the requirements.txt seems out-of-date; the listed version of pydicom does not match the methods that are used (for example, requirements.txt lists 1.2.1, but then in base_client.py, add_private_dict_entries is used which wasn't introduced until 1.3). And the version of scipy is far enough out-of-date that there aren't prebuilt wheels for a modern version of macOS.