At the very least we need to test in Evergreen. We might hit this change from the release notes:

The PyInterpreterState struct has been moved into the “internal” header files (specifically Include/internal/pycore_pystate.h). An opaque PyInterpreterState is still available as part of the public API (and stable ABI). The docs indicate that none of the struct’s fields are public, so we hope no one has been using them. However, if you do rely on one or more of those private fields and have no alternative then please open a BPO issue. We’ll work on helping you adjust (possibly including adding accessor functions to the public API). (See bpo-35886.)

See these changes in psycopg2 and mod_wsgi for examples of the change we may have to make in the bson module:

https://github.com/psycopg/psycopg2/commit/c929f200484f5a2355edf769aaaee1e036129fd8
https://github.com/GrahamDumpleton/mod_wsgi/commit/b03b02df6318afe26052db5b0365732152cacea2

Even if we don't strictly need these changes we should probably use PyInterpreterState_Main when available anyway. It seems to have been added in CPython 3.7.