Merge #10781: Python cleanups

Merge #10781: Python cleanups

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)

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