Execnet

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* Update calls of `threading.Event.isSet()` to `threading.Event.is_set()`, which avoids a deprecation warning with Python 3.10.

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* Dropped support for Python 3.4.

* `118 <https://github.com/pytest-dev/execnet/pull/118>`__: Fixed internal leak that should make
``execnet`` execute remote code in the main thread more often; previously it would sometimes
spawn a thread to execute a ``remote_exec`` call, even when the caller
didn't issue multiple ``remote_exec`` calls at the same time. Some frameworks require code
to execute in the main thread, so the previous behavior would break them on occasion (see
`pytest-dev/pytest-xdist620 <https://github.com/pytest-dev/pytest-xdist/issues/620>`__
for an example).

* `115 <https://github.com/pytest-dev/execnet/pull/115>`__: Current working directory is now
restored when calling ``script/socketserver.py``. The script now also loops by default
when called from the command-line.

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* `108 <https://github.com/pytest-dev/execnet/issues/108>`__: Revert ``linecache`` optimization introduced in ``1.7.0`` which
broke remote execution.

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* `102 <https://github.com/pytest-dev/execnet/pull/102>`__: Show paths in stack traces
generated by ``remote_exec()``.

* `100 <https://github.com/pytest-dev/execnet/pull/100>`__: Fix flaky hangs in ``workerpool.waitall``.

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* `98 <https://github.com/pytest-dev/execnet/pull/98>`__: Internal change to avoid
using deprecated ``funcargs`` name in pytest 5+.

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* ``execnet`` no longer supports Python 2.6 and 3.3 (85). Users of those Python versions
using a recent enough ``pip`` should not be affected, as ``pip`` will only install
``1.5.0`` for them.

* Update test suite to support ``pytest>4``.