Add gunicorn for deployment on Unix systems

requirements.txt

@@ -1,72 +1,3 @@
-attrs==19.3.0
-Automat==0.8.0
-blinker==1.4
-certifi==2019.11.28
-chardet==3.0.4
-Click==7.0
-cloud-init==20.4
-colorama==0.4.3
-command-not-found==0.3
-configobj==5.0.6
-constantly==15.1.0
-cryptography==2.8
-dbus-python==1.2.16
-distro==1.4.0
-distro-info===0.23ubuntu1
-entrypoints==0.3
-Flask==1.1.2
-gunicorn==20.1.0
-httplib2==0.14.0
-hyperlink==19.0.0
-idna==2.8
-importlib-metadata==1.5.0
-incremental==16.10.1
-itsdangerous==1.1.0
-Jinja2==2.10.1
-jsonpatch==1.22
-jsonpointer==2.0
-jsonschema==3.2.0
-keyring==18.0.1
-language-selector==0.1
-launchpadlib==1.10.13
-lazr.restfulclient==0.14.2
-lazr.uri==1.0.3
-MarkupSafe==1.1.0
-more-itertools==4.2.0
+Flask>=1.0,<=1.1.2
 musicbrainzngs==0.7.1
-netifaces==0.10.4
-oauthlib==3.1.0
-pexpect==4.6.0
-pyasn1==0.4.2
-pyasn1-modules==0.2.1
-PyGObject==3.36.0
-PyHamcrest==1.9.0
-PyJWT==1.7.1
-pymacaroons==0.13.0
-PyNaCl==1.3.0
-pyOpenSSL==19.0.0
-pyrsistent==0.15.5
-pyserial==3.4
-python-apt==2.0.0+ubuntu0.20.4.3
-python-debian===0.1.36ubuntu1
-PyYAML==5.3.1
-requests==2.22.0
-requests-unixsocket==0.2.0
-SecretStorage==2.3.1
-service-identity==18.1.0
-simplejson==3.16.0
-six==1.14.0
-sos==4.0
-ssh-import-id==5.10
-systemd-python==234
-Twisted==18.9.0
-ubuntu-advantage-tools==20.3
-ubuntu-drivers-common==0.0.0
-ufw==0.36
-unattended-upgrades==0.1
-urllib3==1.25.8
-wadllib==1.3.3
-Werkzeug==1.0.1
-xkit==0.0.0
-zipp==1.0.0
-zope.interface==4.7.1
+gunicorn; platform_system == "Linux"