Bump reportlab from 2.7 to 3.5.55 in /requirements

Bumps [reportlab](http://www.reportlab.com/) from 2.7 to 3.5.55.

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

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

requirements/requirements.txt

@@ -16,7 +16,7 @@ pisa==3.0.33
 psycopg2==2.6.1
 python-memcached==1.53
 PyYAML==3.11
-reportlab==2.7
+reportlab==3.5.55
 requests==2.8.1
 six==1.10.0
 djangorestframework==2.4.8