78214588d Use for-loop instead of list comprehension (practicalswift)
823979436 Use the variable name _ for unused return values (practicalswift)
2e6080bbf Remove unused variables and/or function calls (practicalswift)
9b94054b7 Avoid reference to undefined name: stderr does not exist, sys.stderr does (practicalswift)
51cb6b822 Use print(...) instead of undefined printf(...) (practicalswift)
25cd520fc Use sys.exit(...) instead of exit(...): exit(...) should not be used in programs (practicalswift)
Pull request description:
Python cleanups:
* Avoid reference to undefined name: `stderr` does not exist, `sys.stderr` does
* Use `print(...)` instead of undefined `printf(...)`
* Avoid redefinition of variable (`tx`) in list comprehension
* Remove unused variables and/or function calls
* Use `sys.exit(...)` instead of `exit(...)`: [`exit(...)` should not be used in programs](https://github.com/bitcoin/bitcoin/pull/10753#discussion_r125935027)
Tree-SHA512: 1238dfbc1d20f7edadea5e5406a589f293065638f6234809f0d5b6ba746dffe3d276bc5884c7af388a6c798c61a8759faaccf57f381225644754c0f61914eb4b
