Bump protobuf from 3.7.0 to 3.15.0 in /python-example

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.7.0 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@v3.7.0...v3.15.0)

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

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

Author: dependabot[bot]

python-example/Pipfile

@@ -5,7 +5,7 @@ verify_ssl = true
 
 [packages]
 pitayaserver = "==0.3.0"
-protobuf = "==3.7.0"
+protobuf = "==3.15.0"
 six = "==1.12.0"
 uuid = "==1.30"
 

python-example/Pipfile.lock

@@ -1,4 +1,25 @@
+#
+# These requirements were autogenerated by pipenv
+# To regenerate from the project's Pipfile, run:
+#
+#    pipenv lock --requirements --dev
+#
+
+# Note: in pipenv 2020.x, "--dev" changed to emit both default and development
+# requirements. To emit only development requirements, pass "--dev-only".
+
+-i https://pypi.org/simple/
+astroid==2.9.3; python_full_version >= '3.6.2'
+isort==5.10.1; python_version < '4.0' and python_full_version >= '3.6.1'
+lazy-object-proxy==1.7.1; python_version >= '3.6'
+mccabe==0.6.1
 pitayaserver==0.3.0
-protobuf==3.7.0
+platformdirs==2.5.1; python_version >= '3.7'
+protobuf==3.15.0
+pylint==2.12.2
+setuptools==60.10.0; python_version >= '3.7'
 six==1.12.0
+toml==0.10.2; python_version >= '2.6' and python_version not in '3.0, 3.1, 3.2, 3.3'
+typing-extensions==4.1.1; python_version < '3.10'
 uuid==1.30
+wrapt==1.13.3; python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'