[FIX] reqs: bump up recommended requests version

In light of CVE-2018-18074, using python-requests 2.20.0 is recommended,
or any distribution-specific version that was patched.

See also https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-18074

requirements.txt

@@ -30,7 +30,7 @@ pyusb==1.0.0
 PyYAML==3.12
 qrcode==5.3
 reportlab==3.3.0
-requests==2.11.1
+requests==2.20.0
 six==1.10.0
 suds-jurko==0.6
 vatnumber==1.2