Pytest-xprocess

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- drop support for python 3.5 and 3.6
- reorganize internals. ``pytest-xprocess`` is now a package and all resources
used by running processes are kept as instances of ``XProcessResources``.

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- Fix bug with previous release where internal module was missing

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- `ProcessInfo.terminate` will now terminate outer leaves in process
tree first and work its way towards root process. For example, if a process
has child and grandchild, xprocess will terminate first child and grandchild
and only then will the root process receive a termination signal.

- `ProcessStarter` now has attr:`terminate_on_interrupt`. This flag will
make xprocess attempt to terminate and clean up all started process resources
upon interruptions during pytest runs (`CTRL+C`, `SIGINT` and internal errors)
when set to `True`. It will default to `False`, so if the described behaviour
is desired the flag must be explicitly set `True`.

- Add a new `popen_kwargs` variable to `ProcessStarter`, this variable can
be used for passing keyword values to the `subprocess.Popen` constructor,
giving the user more control over how the process is initialized.

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- Fix `ResourceWarning` in :meth:`XProcess.ensure` caused by not properly
waiting on process exit and leaked File handles

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- `ProcessStarter` now has :meth:`startup_check`. This method can be optionaly overridden and will be called upon to check process responsiveness
after :attr:`ProcessStarter.pattern` is matched. By default, :meth:`XProcess.ensure` will only attempt to match :attr:`ProcessStarter.pattern` when starting a process, if matched, xprocess
will consider the process as ready to answer queries. If :meth:`startup_check` is provided though, its return value will also be considered to determine if the process has been
successfully started. If :meth:`startup_check` returns `True` after :attr:`ProcessStarter.pattern` has been matched, :meth:`XProcess.ensure` will return sucessfully. In contrast, if
:meth:`startup_check` does not return `True` before timing out, :meth:`XProcess.ensure` will raise a `TimeoutError` exception.
- Remove deprecated :meth:`xprocess.CompatStarter`

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- `ProcessStarter` now has a new `timeout` class variable optionaly overridden to define the maximum time :meth:`xprocess.ensure` should wait for process output when trying to match :attr:`ProcessStarter.pattern`. Defaults to 120 seconds.
- The number of lines in the process logfile watched for :attr:`ProcessStarter.pattern` is now configurable and can be changed by setting :attr:`ProcessStarter.max_read_lines` to the desired value. Defaults to 50 lines.
- Make :meth:`XProcessInfo.isrunning` ignore zombie processes by default. Pass ``ignore_zombies=False`` to get the previous behavior, which was to consider zombie processes as running.