Merge pull request #1917 from dalf/requirements-upgrade

[mod] requirements.txt: upgrade dependency versions.

requirements.txt

-certifi==2019.3.9
+certifi==2020.4.5.1
 babel==2.7.0
 flask-babel==1.0.0
-flask==1.0.2
-idna==2.8
-jinja2==2.10.1
-lxml==4.3.3
+flask==1.1.2
+idna==2.9
+jinja2==2.11.1
+lxml==4.5.0
 pygments==2.1.3
-pyopenssl==19.0.0
+pyopenssl==19.1.0
 python-dateutil==2.8.0
-pyyaml==5.1
-requests[socks]==2.22.0
+pyyaml==5.3.1
+requests[socks]==2.23.0