www/py-urlwatch: Attempt to fix intermittent build issues

• If the DOCS option is enabled, the build process occasionally fails. Attempt to fix this issue by setting MAKE_JOBS_UNSAFE accordingly.

• No PORTREVISION bump as the package content isn't changed.

Reported by: pkg-fallout
MFH: 2022Q3

(cherry picked from commit 3f08f1372c1f4e6384e6b3f8683582b88dd007ac)