sshuttle: Importlib has no attribute util

[mweinelt]

Describe the bug
Sshuttle since 1.0.2 on master does not work, it immediately crashes with the following traceback:

Traceback (most recent call last):
  File "/nix/store/ln7wczj2q10w410bx1c0y00jscyyw22s-sshuttle-1.0.2/bin/..sshuttle-wrapped-wrapped", line 9, in <module>
    sys.exit(main())
  File "/nix/store/ln7wczj2q10w410bx1c0y00jscyyw22s-sshuttle-1.0.2/lib/python3.8/site-packages/sshuttle/cmdline.py", line 85, in main
    return_code = client.main(ipport_v6, ipport_v4,
  File "/nix/store/ln7wczj2q10w410bx1c0y00jscyyw22s-sshuttle-1.0.2/lib/python3.8/site-packages/sshuttle/client.py", line 793, in main
    return _main(tcp_listener, udp_listener, fw, ssh_cmd, remotename,
  File "/nix/store/ln7wczj2q10w410bx1c0y00jscyyw22s-sshuttle-1.0.2/lib/python3.8/site-packages/sshuttle/client.py", line 447, in _main
    (serverproc, serversock) = ssh.connect(
  File "/nix/store/ln7wczj2q10w410bx1c0y00jscyyw22s-sshuttle-1.0.2/lib/python3.8/site-packages/sshuttle/ssh.py", line 92, in connect
    content = get_module_source('sshuttle.assembler')
  File "/nix/store/ln7wczj2q10w410bx1c0y00jscyyw22s-sshuttle-1.0.2/lib/python3.8/site-packages/sshuttle/ssh.py", line 18, in get_module_source
    spec = importlib.util.find_spec(name)
AttributeError: module 'importlib' has no attribute 'util'

To Reproduce
Steps to reproduce the behavior:

1. sshuttle -r you@jumphost 192.0.2.0/24

Expected behavior
It shouldn't crash.

Notify maintainers

@domenkozar @carlosdagos

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.7.9, NixOS, 20.09.git.3115fe6989f (Nightingale)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.7`
 - channels(root): `"nixos-20.09pre234242.c71518e75bf, nixos-hardware, nixos-unstable-20.09pre234242.c71518e75bf"`
 - channels(hexa): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: sshuttle
# a list of nixos modules affected by the problem
module: