Chameleon

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- Allow setting a custom value representation function, allowing
custom formatting of variables during exception formatting.

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- Fix compatibility issue with Python 3.9.

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- Fixed garbage collection issue with variable scope objects (issue 301).

- Fixed issue where setting a global variable would not be available
locally.

- A `RepeatDict` no longer inherits from `dict` since it does not
actually provide that interface in a meaningful way.

- Added feature gate `enable_comment_interpolation` which controls
whether expression interpolation is enabled inside HTML comments
(default is enabled).

- Added support for Python 3.6+ f-strings (issue 294).

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- Fix SyntaxWarnings in Python 3.8 resulting from comparing literals with 'is'.
See https://github.com/plone/Products.CMFPlone/issues/2890.

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- Fix limited search expression for illegal double hyphens in HTML
comments to fix issue 289.

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- Exclude `RuntimeError` (or `RecursionError` when available) from
exception wrapping.

- Fix double dollar '$$' escaping such that a double dollar is always
resolved, either as an interpolation expression, or as an escape
where it is substituted by a single dollar symbol. This is now
consistent with Zope's handling of this character.

Backslash-escaping of dollar-based string interpolation is no longer
supported. The documentation has been updated to reflect this
change.

This fixes issue 283. Note that this reverses some of the changes
introduced to fix issue 265.

- Drop support for Python 3.3.