Better support for trace-function switching

line_profiler/_line_profiler.pyx
    _LineProfilerManager
        - Renamed from `_ThreadState`
        - Added init argument and property `set_frame_local_trace`
        - Added `.__call__()` so that other Python code doesn't crash
          when it temporarily overrides and restores the `sys` trace
          callback via `sys.gettrace()` and `sys.settrace()`
    LineProfiler
        - Updated docstring
        - Added init argument and property `set_frame_local_trace`
        - Renamed:
          - `_all_thread_states` -> `_managers`
          - `_thread_state` -> `_manager`
    python_trace_callback()
        - Renamed argument `state_` -> `manager_`
        - Now setting frame's `.f_trace` if entering a frame with the
          `PyTrace_CALL` event and if `manager_.set_frame_local_trace`
          is true

tests/test_sys_trace.py
    <General>
        Fixed lints in some tests
    @isolate_test_in_subproc
        Now taking an optional argument `debug` to permit using `pytest`
        instead of `python` to run the test, which provides more helpful
        output in case of a failure (but is significantly slower)
    test_python_level_trace_manipulation()
        New test for ensuring that `LineProfiler` plays well with
        Python-level trace-function manipulation via `sys.gettrace()`
        and `sys.settrace()`

Author: TTsangSC