Util: Move the extend_form import to the only function using it

This breaks a circular import issue.
2 jobs for booleans in 30 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #16524
run-test

00:00:30

 
  Build
skipped #16525
run-build
 
Name Stage Failure
failed
run-test Test
Successfully installed astroid-2.3.3 isort-4.3.21 lazy-object-proxy-1.4.3 mccabe-0.6.1 pylint-2.4.4 six-1.14.0 wrapt-1.11.2
$ python3 -m pylint nbxmpp --disable=C0103,C0201,C0301,C0326,C0330,W0201,W0212,W0221,W0231,W0233,W0621,W0622,R0201,E1101,E1135
************* Module nbxmpp.util
nbxmpp/util.py:349:4: C0415: Import outside toplevel (nbxmpp.modules.dataforms) (import-outside-toplevel)

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Your code has been rated at 10.00/10 (previous run: 10.00/10, -0.00)

ERROR: Job failed: exit status 1