rsa_keys: rename variable which shadows an imported module
rsa_keys: rename variable which shadows an imported module

The hmac primitive was imported from pyca/cryptography and later shadowed by a
local variable called hmac, requiring references to the pyca/cryptography
primitive to be fully namespaced. Rename the local variable so that the import
is no longer shadowed.

Signed-off-by: Joshua Lock <edc229f50a11a11cdece0270af6320fb299dca1e@vmware.com>

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib