Bump protobuf from 3.6.0 to 3.15.0

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.6.0 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.6.0...v3.15.0)

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

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

Author: dependabot[bot]

requirements.txt

@@ -1,2 +1,2 @@
 grpcio==1.18.0
-protobuf==3.6.0
+protobuf==3.15.0

setup.py

@@ -11,6 +11,6 @@
       packages=['iosxr_grpc'],
       install_requires=[
           'grpcio==1.18.0',
-          'protobuf==3.6.0',
+          'protobuf==3.15.0',
       ],
       zip_safe=False)