Bump requests minimal version

It's a follow-up of [1].
JSONDecodeError exception was implemented in requests
since 2.27.0 [2].

[1] https://review.opendev.org/c/openstack/python-openstackclient/+/924927
[2] psf/requests@db575ee

Change-Id: I8a6a8ac064cb0703097ed686b1fcd2983181fcfa

Author: joelcapitao

requirements.txt

@@ -12,5 +12,5 @@ osc-lib>=2.3.0 # Apache-2.0
 oslo.i18n>=3.15.3 # Apache-2.0
 python-keystoneclient>=3.22.0 # Apache-2.0
 python-cinderclient>=3.3.0 # Apache-2.0
-requests>=2.14.2 # Apache-2.0
+requests>=2.27.0 # Apache-2.0
 stevedore>=2.0.1 # Apache-2.0