Add `form` to PredicateBuilder. #363

Job	Run time
Test (3.13, 18.x, 2.9, ubuntu-latest)	1m 1s
Test Mountebank versions (3.13, 16.x, 2.8, ubuntu-latest)	1m 11s
Test Python versions (3.12, 18.x, 2.9, ubuntu-latest, false)	53s
Test Mountebank versions (3.13, 16.x, 2.0, ubuntu-latest)	1m 6s
Check coverage (3.13, 18.x, 2.9, ubuntu-latest)	1m 2s
Linting and docs (3.13, ubuntu-latest)	1m 53s
Test Python versions (3.11, 18.x, 2.9, ubuntu-latest, false)	49s
Test Mountebank versions (3.13, 16.x, 1.16, ubuntu-latest)	1m 0s
Test Python versions (3.10, 18.x, 2.9, ubuntu-latest, false)	49s
Test OSes (3.13, 18.x, 2.9, windows-latest)	3m 53s
Test Python versions (3.9, 18.x, 2.9, ubuntu-latest, false)	57s
Test OSes (3.13, 18.x, 2.9, macos-latest)	41s
Test Python versions (pypy-3.10, 18.x, 2.9, ubuntu-latest, false)	1m 16s
Test Python versions (3.13.0-alpha - 3.13.0, true, 18.x, 2.9, ubuntu-latest)	1m 14s
	17m 45s

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