PYTHON-6157 Prep for pymongocrypt 1.20.0 release

[aclark4life]

https://jira.mongodb.org/browse/PYTHON-6157

Changes in this PR

Bump __version__ from 1.20.0.dev0 to 1.20.0 in preparation for the release.

Test Plan

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?