Merge #18210: test: type hints in Python tests

bd7e530f010d43816bb05d6f1590d1cd36cdaa2c This PR adds initial support for type hints checking in python scripts. (Kiminuo)

Pull request description:

  This PR adds initial support for type hints checking in python scripts.

  Support for type hints was introduced in Python 3.5. Type hints make it easier to read and review code in my opinion. Also an IDE may discover a potential bug sooner. Yet, as PEP 484 says: "It should also be emphasized that Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention."

  [Mypy](https://mypy.readthedocs.io/en/latest/index.html) is used in `lint-python.sh` to do the type checking. The package is standard so there is little chance that it will be abandoned. Mypy checks that type hints in source code are correct when they are not, it fails with an error.

  **Notes:**

  * [--ignore-missing-imports](https://mypy.readthedocs.io/en/latest/command_line.html#cmdoption-mypy-ignore-missing-imports) switch is passed on to `mypy` checker for now. The effect of this is that one does not need `# type: ignore` for `import zmq`. More information about import processing can be found [here](https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports). This can be changed in a follow-up PR, if it is deemed useful.
  * We are stuck with Python 3.5 until 04/2021 (see https://packages.ubuntu.com/xenial/python3). When Python version is bumped to 3.6+, one can change:
      ```python
      _opcode_instances = []  # type: List[CScriptOp]
      ```
      to
      ```python
      _opcode_instances:List[CScriptOp] = []
      ```
      for type hints that are **not** function parameters and function return types.

  **Useful resources:**

  * https://docs.python.org/3.5/library/typing.html
  * https://www.python.org/dev/peps/pep-0484/

ACKs for top commit:
  fanquake:
    ACK bd7e530f010d43816bb05d6f1590d1cd36cdaa2c - the type checking is not the most robust (there are things it fails to detect), but I think this is worth adopting (in a limited capacity while we maintain 3.5 compat).
  MarcoFalke:
    ACK bd7e530f010d43816bb05d6f1590d1cd36cdaa2c fine with me

Tree-SHA512: 21ef213915fb1dec6012f59ef17484e6c9e0abf542a316b63d5f21a7778ad5ebabf8961ef5fc8e5414726c2ee9c6ae07c7353fb4dd337f8fcef5791199c8987a

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