Upgrade jinja2 to fix issue installing old markupsafe on Ubuntu 18.04.

This also aligns with our requirement of Python 3.6 or greater due to recent
jinja2 versions requiring f-strings.

docs/source/siteadmin/relnotes.rst

@@ -22,6 +22,13 @@ This chapter has important information about our current and previous releases.
 0.13.0 (Work in progress)
 =========================
 
+**Breaking changes:**
+
+- MediaGoblin now supports a minimum of Python 3.6 due to use of f-strings
+  internally and updated versions of required Python packages (Ben Sturmfels)
+
+**Changes:**
+
 - Extend left/right arrow key navigation to paginated views [trac#5627] (Olivier Mehani)
 - Fix incorrect setuptools install location for db/migrations/env.py (Elisei Roca)
 - Set videos to preload="metadata" to prevent upfront download [trac#5625] (Michael McMahon)

setup.cfg

@@ -36,6 +36,7 @@ classifiers=[
 
 [options]
 packages = find:
+# We now use f-strings internally and allow Python requirements do too.
 python_requires = >=3.6
 zip_safe = False
 include_package_data = True
@@ -50,7 +51,7 @@ install_requires =
     ExifRead>=2.0.0
     feedgenerator
     itsdangerous
-    jinja2<3.0.0  # 3.0.0 uses f-strings (Python >= 3.7) breaking Debian 9.
+    jinja2
     jsonschema
     Markdown
     oauthlib