Maintenance release

This release fixes issues introduced with 3.6.2 and the extend backport.
Also fixes an issue with memory lifespan of error_src on the C-API.
Additionally some edge case crashes have also been addressed.

Changelog

• Fix compound extend warning (@mgreter) #2990,
• Fix extend being stuck in endless loop (@mgreter) #3007
• Fix various edge-case segfault crashes (@mgreter) #3027
• Extend error_src lifetime on c-api context (@Taritsyn) #3022
• Fix memory leak in permutation function (@mgreter) #3015
• Preserve indentation in nested mode (@mgreter) #3021