Add preliminary support for Python 3.14

[icemac]

Problems:

• z3c.pt using no longer existing ast.Str – Configuring for pure-python z3c.pt#32
• zope.pagetemplate using no longer existing ast.Str it is not used there at all
• Zope itself uses no longer existing ast.Str
• Tests break because ast.Str is still used – chameleon 4.4.4 seems to be the reason

Other compatibility issues that may cause problems are captured in #1259.

[icemac]

I have no idea where this ast.Str comes from: I searched zopefoundation and Chameleon 4.4.4 which is still used – no success.

So I am giving up here now. Anyone to take over?

[dataflake]

ast.Str comes from Chameleon 4.4.4. The references were fixed in version 4.6.0. We cannot move to that version until malthe/chameleon#438 is fixed - unfortunately I have not gotten any response at all.