Various nits in wallet.py
Various nits in wallet.py

Summary:
As per title.

Depends on D1162

Test Plan:
  ./test/functional/test_runner.py wallet

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1163

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