Bump pymongo from 3.12.1 to 4.6.3 in /imagegw

Bumps [pymongo](https://github.com/mongodb/mongo-python-driver) from 3.12.1 to 4.6.3.
- [Release notes](https://github.com/mongodb/mongo-python-driver/releases)
- [Changelog](https://github.com/mongodb/mongo-python-driver/blob/master/doc/changelog.rst)
- [Commits](mongodb/mongo-python-driver@3.12.1...4.6.3)

---
updated-dependencies:
- dependency-name: pymongo
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

imagegw/requirements.txt

@@ -1,3 +1,3 @@
-pymongo==3.12.1
+pymongo==4.6.3
 sanic==20.9.1
 pylint