Bump protobuf from 3.12.2 to 3.18.3 in /binder

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.12.2 to 3.18.3.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.12.2...v3.18.3)

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

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

Author: dependabot[bot]

binder/requirements.txt

@@ -13,7 +13,7 @@ Markdown==3.2.2
 numpy==1.19.5
 opt-einsum==3.2.1
 pandas==0.24.2
-protobuf==3.12.2
+protobuf==3.18.3
 python-dateutil==2.8.1
 pytz==2020.1
 six==1.15.0