www/mitmproxy: Unbreak at runtime

• After r511877 (initial cause, already fixed in r518596) and r516400 [1] the port was broken at runtime. Unbreak it by relaxing the version range for net/py-ldap3.
  
  Also relax the version range for devel/py-urwid which was recently updated in r519201.

• Adjust the RUN_DEPENDS by making them more specific (as given in setup.py) to reduce the likelihood of runtime errors in the future.

• Bump PORTREVISION due changed code.

A MFH is not required as www/mitmproxy in the 2019Q4 branch isn't affected
and was already fixed with r518597.

PR: 242025
Reported by: ale [1]
Approved by: maintainer timeout (2 weeks)