url preview: Add python-magic dependency for content-type sniffing.

requirements/common.in

@@ -173,6 +173,7 @@ ijson==2.3
 # Needed for link preview
 beautifulsoup4==4.7.1
 pyoembed==0.1.2
+python-magic==0.4.15
 
 # The Zulip API bindings, from its own repository.  We integrate with
 # these tightly, including fetching content not included in the normal

requirements/dev.txt

@@ -143,6 +143,7 @@ python-digitalocean==1.14.0
 python-gcm==0.4
 python-jose==2.0.2        # via moto
 python-ldap==3.2.0        # via django-auth-ldap, pyldap
+python-magic==0.4.15
 python-twitter==3.5
 python3-openid==3.1.0     # via social-auth-core
 pytz==2019.1

requirements/prod.txt

@@ -102,6 +102,7 @@ pysocks==1.6.8            # via twilio
 python-dateutil==2.8.0
 python-gcm==0.4
 python-ldap==3.2.0        # via django-auth-ldap
+python-magic==0.4.15
 python-twitter==3.5
 python3-openid==3.1.0     # via social-auth-core
 pytz==2019.1

scripts/lib/setup_venv.py

@@ -39,6 +39,7 @@ VENV_DEPENDENCIES = [
     "libxslt1-dev",         # Used for installing talon
     "libpq-dev",            # Needed by psycopg2
     "libssl-dev",           # Needed to build pycurl and other libraries
+    "libmagic1",            # Used for install python-magic
 
     # This is technically a node dependency, but we add it here
     # because we don't have another place that we install apt packages

version.py

@@ -11,4 +11,4 @@ LATEST_RELEASE_ANNOUNCEMENT = "https://blog.zulip.org/2019/03/01/zulip-2-0-relea
 # Typically, adding a dependency only requires a minor version bump, and
 # removing a dependency requires a major version bump.
 
-PROVISION_VERSION = '30.4'
+PROVISION_VERSION = '31.0'