pybind/mgr/dashboard: bump up requests to 2.25.1

pybind/mgr/dashboard: bump up requests to 2.25.1

request 2.20 is not compatible with urllib3 v1.25.2 and up. this causes
trouble of incompatibility with other python modules. for instance, we
now have following error:

ERROR: pip's dependency resolver does not currently take into account
all the packages that are installed. This behaviour is the source of the
following dependency conflicts.
botocore 1.20.14 requires urllib3<1.27,>=1.25.4, but you have urllib3
1.24.3 which is incompatible.

see also https://github.com/psf/requests/pull/5092

Signed-off-by: Kefu Chai <kchai@redhat.com>