mgr: replace deprecated PyImport_ImportModuleNoBlock with PyImport_ImportModule

mgr: replace deprecated PyImport_ImportModuleNoBlock with PyImport_ImportModule

PyImport_ImportModuleNoBlock was deprecated in Python 3.13:

  PyModule.cc:374:21: warning: 'PyImport_ImportModuleNoBlock' is deprecated
  [-Wdeprecated-declarations]

The "no block" variant was introduced to avoid deadlocking on Python 2's
single global import lock, but Python 3's per-module locking (since
3.3) made the distinction meaningless. PyImport_ImportModule is the
direct replacement and is compatible with all Python versions.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>