Bump protobuf from 3.14.0 to 3.15.6

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.14.0 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.14.0...v3.15.6)

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

requirements.in

@@ -4,10 +4,10 @@ cffi==1.14.4
 future==0.18.2
 itsdangerous==1.1.0
 pbr==1.10.0
-protobuf==3.14.0
+protobuf==3.15.6
 pyopenssl==19.1.0
 PyYAML==5.2 # PyYAML 5.3 does not support Python 3.4
-pytz==2020.4
+pytz==2021.1
 requests>=2.20.0
 urllib3>=1.24.3
 deprecated==1.2.10

requirements.txt

@@ -4,24 +4,54 @@
 #
 #    pip-compile --output-file=requirements.txt requirements.in
 #
-asn1==2.2.0               # via -r requirements.in
-certifi==2018.11.29       # via requests
-cffi==1.14.4              # via -r requirements.in, cryptography
-chardet==3.0.4            # via requests
-cryptography==2.8         # via -r requirements.in, pyopenssl
-deprecated==1.2.10        # via -r requirements.in
-future==0.18.2            # via -r requirements.in
-idna==2.7                 # via requests
-iso8601==0.1.13           # via -r requirements.in
-itsdangerous==1.1.0       # via -r requirements.in
-pbr==1.10.0               # via -r requirements.in
-protobuf==3.14.0          # via -r requirements.in
-pycparser==2.18           # via cffi
-pyopenssl==19.1.0         # via -r requirements.in
-pytz==2020.4              # via -r requirements.in
-pyyaml==5.2               # via -r requirements.in
-requests==2.21.0          # via -r requirements.in
-six==1.10.0               # via cryptography, protobuf, pyopenssl
-urllib3==1.24.3           # via -r requirements.in, requests
-wheel==0.33.6             # via -r requirements.in
-wrapt==1.11.2             # via deprecated
+asn1==2.2.0
+    # via -r requirements.in
+certifi==2018.11.29
+    # via requests
+cffi==1.14.4
+    # via
+    #   -r requirements.in
+    #   cryptography
+chardet==3.0.4
+    # via requests
+cryptography==2.8
+    # via
+    #   -r requirements.in
+    #   pyopenssl
+deprecated==1.2.10
+    # via -r requirements.in
+future==0.18.2
+    # via -r requirements.in
+idna==2.7
+    # via requests
+iso8601==0.1.13
+    # via -r requirements.in
+itsdangerous==1.1.0
+    # via -r requirements.in
+pbr==1.10.0
+    # via -r requirements.in
+protobuf==3.15.6
+    # via -r requirements.in
+pycparser==2.18
+    # via cffi
+pyopenssl==19.1.0
+    # via -r requirements.in
+pytz==2021.1
+    # via -r requirements.in
+pyyaml==5.2
+    # via -r requirements.in
+requests==2.21.0
+    # via -r requirements.in
+six==1.10.0
+    # via
+    #   cryptography
+    #   protobuf
+    #   pyopenssl
+urllib3==1.24.3
+    # via
+    #   -r requirements.in
+    #   requests
+wheel==0.33.6
+    # via -r requirements.in
+wrapt==1.11.2
+    # via deprecated