Bump btrees from 5.0 to 5.1

Bumps [btrees](https://github.com/zopefoundation/BTrees) from 5.0 to 5.1.
- [Changelog](https://github.com/zopefoundation/BTrees/blob/master/CHANGES.rst)
- [Commits](zopefoundation/BTrees@5.0...5.1)

---
updated-dependencies:
- dependency-name: btrees
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,4 +1,4 @@
-BTrees==5.0
+BTrees==5.1
 cffi==1.16.0
 dnspython==2.4.2
 mysql-connector==2.2.9