Add changelog entries

Signed-off-by: Bernát Gábor <[email protected]>

Author: gaborbernat

docs/changelog/1776.bugfix.rst

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+Entries in the ``set_env`` does not reference environments from ``set_env`` - by :user:`gaborbernat`.

docs/changelog/1779.bugfix.rst

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+``env`` substitution does not uses values from ``set_env`` - by :user:`gaborbernat`.

docs/changelog/1779.feature.rst

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+Raise exception when set env enters into a circular reference - by :user:`gaborbernat`.

docs/changelog/1784.bugfix.rst

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+- Python version markers are stripped in package dependencies (after wrongfully being detected as an extra marker).
+- In packaging APIs do not set ``PYTHONPATH`` (to empty string) if ``backend-path`` is empty.
+- Fix commands parsing on Windows (do not auto-escape ``\`` - instead users should use the new ``{\}``, and on parsed
+  arguments strip both ``'`` and ``"`` quoted outcomes).
+- Allow windows paths in substitution set/default (the ``:`` character used to separate substitution arguments may
+  also be present in paths on Windows - do not support single capital letter values as substitution arguments) -
+  by :user:`gaborbernat`.

docs/changelog/1784.feature.rst

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+- Raise exception when variable substitution enters into a circle.
+- Add ``{/}`` as substitution for os specific path separator.
+- Add ``{env_bin_dir}`` constant substitution.
+- Implement support for ``--discover`` flag - by :user:`gaborbernat`.