Change "client" to "SDK" in README

[jseldess]

Problem

We are inconsistent in how we refer to our client libraries.

Solution

As we mostly refer to our "SDKs" rather than our "clients", we've updated our docs to prefer "SDK". This PR makes equivalent updates to the Python README.

Addresses: https://app.asana.com/0/1206419570114940/1207978475519546/f

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Infrastructure change (CI configs, etc)
• [x] Non-code change (docs, etc)
• [ ] None of the above: (explain here)

Test Plan

Describe specific steps for validating this change.