feat: SystemIdentityMixin reads a host's SMBIOS identity (4.9.0)

What ties a server to its BMC: the BMC reports the server's serial
number and system UUID, the host reads the same values from its DMI
tables. Both drivers that read DMI (napalm-linux, napalm-proxmox) had a
copy of the shell for it, and neither read the board serial.

SystemIdentityMixin.get_system_identity() reads product_uuid,
product_serial and board_serial in one framed command and returns them
with vendor placeholders empty: a Supermicro board's product serial is
0123456789 on every one of them, others read "To be filled by O.E.M."
or "Default string", and many cheap boards share the UUID
03000200-0400-0500-0006-000700080009. is_placeholder_identity() is the
same filter for a BMC driver's side. A driver supplies only
_run_system_identity_command(command, privileged=); the files are
root's.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "napalm-device-types"
-version = "4.8.0"
+version = "4.9.0"
 description = "Abstract device-type base classes for NAPALM drivers"
 readme = "README.md"
 requires-python = ">=3.10"