fix: IPMI polls after the first failed; BMC drivers collect SNMP health (0.2.1)

The second IPMI poll of a BMC in the same worker process failed with
"'<' not supported between instances of 'float' and 'NoneType'".
pyghmi files a session in Session.initting_sessions under the
credentials as str and deletes it only under bytes, so the entry stays
and the next Command got the old, logged-out session back. The driver
drops that entry itself before open() and after close(). Reproduced and
verified live against the Supermicro X10DRL-i: three polls in one
process now succeed.

IpmiDriver and RedfishDriver mix in HealthMetricsMixin, so netOrk's
SNMP health poll reads the BMC's own load, memory and interfaces where
the BMC runs net-snmp with the UCD-MIB, as the X10's does.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "napalm-redfish"
-version = "0.2.0"
+version = "0.2.1"
 description = "NAPALM driver for server baseboard management controllers over DMTF Redfish"
 readme = "README.md"
 license = { file = "LICENSE" }