fix sugarise-doc-comments.py shebang
add missing $(CFG_PYTHON) prefixes
Change "interface" to "trait" in rustdoc

Closes #4337
Merge pull request #4336 from thestinger/python

fix build for when 'python' is python3
Add test cases (one xfailed, one not)

as per #3601 and #3609
Actually xfail this, unbreak build
Add xfailed test for #3707
clean up condition module:
* use Drop trait in Guard
* fix formatting
* remove bare function in Condition.raise_default
* use moved types instead of borrowed pointers in Condition.{raise,trap}
* import common functions
Add is_mutbl field to arg
Parse `mut` in arguments
Add is_mutbl field to def_arg
Implement `mut` in arguments