Membership, choose_identity, pendulum, asynctest dependencies

• #88 (closed): Add membership command
• #251 (closed): Add pendulum dependency
• Introduce asynctest as dev dependency for Py < 3.8
• #140 (closed): Change argument name for better --help
• #140 (closed): Add choose_identity()
  • Ability to specify an uid or a pubkey to WoT commands (1/2)
  • Ability to choose an identity among multiple maches
• [test] tx.transaction_amount(): monkeypatch ud_value
• Update black to v20.8b1

Close #338 (closed), #251 (closed), #140 (closed), #88 (closed).