Add changelog fragments

docs/changelog-fragments/576.bugfix.rst

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+|project| no longer crashes when received EOF or when channel is not explicitly
+closed -- by :user:`pbrezina`.
+
+Previously, |project| crashed if ``channel.recv`` was called and ``libssh``
+returned ``SSH_EOF`` error. It also crashed on some special occasions where
+channel was not explicitly closed and the session object was garbage-collected
+first.

docs/changelog-fragments/576.doc.rst

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+Fixed the example of invoking remote commands by using
+``Channel.exec_command()`` in snippets -- by :user:`pbrezina`.
+
+Its previously showcased version wasn't functional.

docs/changelog-fragments/576.feature.rst

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+The ``request_exec()`` method was added to the ``Channel`` class. It exposes an
+interface for calling the respective low-level C-API of the underlying
+``libssh`` library -- by :user:`pbrezina`.
+
+Additionally, the following calls to ``libssh`` are now available in the same
+class: ``request_exec()``, ``send_eof()``, ``request_send_signal()`` and
+``is_eof`` which is exposed as a :py:class:`property`.