Chameleon

------------------

- Avoid various traceback reference cycles.

------------------

- Removed compatibility code for unsupported Python versions

- Dropped support for obsolete Python 3.4

- Fixed namespace handling when redefining a variable in the same ``tal:define``
(`237 <https://github.com/malthe/chameleon/issues/237>`_)

- Fixed failure computing a template's digest if ``template.filename`` is None
(`254 <https://github.com/malthe/chameleon/issues/254>`_)

------------------

- Added code optimization to reduce sequential appends of static text.

- The `default` symbol in dynamic attributes is now
symbolic. Previously, it was assigned the string value of the
default attribute text. A similar change has been made for
switch/case expressions.

- The built-in `attrs` dictionary of static element attributes now
correctly works with `tal:define`, etc.

- Fix slice code generation compatibility issue on Python 3.9.

------------------

- Expose default marker as importable symbol
`chameleon.tales.DEFAULT_MARKER`.

- Removed legacy flag `literal_false`. To get a similar behavior, use
`boolean_attributes`.

------------------

- Fix brown-bag release.

------------------

- Fix regression introduced in 3.6.2 where the default marker would
incorrectly change its value between templates, causing issues in
software which depends on the value being treated as a global
object.