refactor: use pylint and fix warnings

[henryiii]

Fixing lots of things. Better error messages, cleaner, etc. A few remaining warnings to fix, not at 10/10 yet.

• Using guard style everywhere
• Reducing local imports
• Reducing local class usage (yuck!)
• Shorter tracebacks (like Python 2) in quite a few cases
• More/better super usage
• Added encoding= setting for open (local, remote isn't supported by Paramiko)
• Use UTF-8 encoding in more places (better future compat)
• Use f-strings everywhere
• Less overlapping names / definitions
• Removed a Python 2.5 workaround :)
• Use staticmethod a few places
• Use sets a few more places
• Remove deprecated abstractproperty usage

[coveralls]

Coverage increased (+0.08%) to 83.527% when pulling fded0debd60362ec3fe9b92ff2d3f2476fbd0549 on henryiii/chore/py2cu into e6fe1a95ee7e18268941296494cc89713fc8e5d3 on master.

[henryiii]

Next I need to work on these:

FAILED tests/test_local.py::TestLocalMachine::test_redirection - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <_io.FileIO [closed]>
FAILED tests/test_local.py::TestLocalMachine::test_iter_lines_timeout_by_type - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <function Popen.__del__ at 0x10e1474c0>
FAILED tests/test_local.py::TestLocalMachine::test_iter_lines_line_timeout - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <function Popen.__del__ at 0x10e1474c0>
FAILED tests/test_local.py::TestLocalMachine::test_logger_pipe - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <_io.FileIO [closed]>
FAILED tests/test_local.py::TestLocalMachine::test_logger_pipe_line_timeout - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <function Popen.__del__ at 0x10e1474c0>
FAILED tests/test_local.py::TestLocalMachine::test_pipeline_stdin - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <_io.FileIO [closed]>
FAILED tests/test_putty.py::TestPuttyMachine::test_putty_command[default] - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <function Popen.__del__ at 0x10e1474c0>

But that's for another day.