protocols(mh): make NTPTimestamp and PMIPv6Timestamp Info classes #1886

Following the owner's ruling on #1869: NTPTimestamp and PMIPv6Timestamp (pcapkit/protocols/internet/mh.py:538, :549) become Info classes, as other nested values in the data models already are. VersionInfo, ModuleDescriptor and TCPSegment stay named tuples.

Breaking. NTPTimestamp has shipped as a tuple since 1.0.1.post2, through every 1.4.x release. PMIPv6Timestamp is beta-only (#437). Once they are Info classes, both lose indexing, unpacking, equality with a plain tuple, hashing, ordering and _replace/_asdict. Attribute access (.seconds, .fraction) and pickling still work. The dumpers will write {"seconds": …, "fraction": …} where they now write {"type": "NTPTimestamp", "value": [s, f]}.

Work:

• Convert both classes. Model them on an existing small Info such as hip.Lifetime (protocols/data/internet/hip.py:559).
• Rewrite mh.py:8985 (seconds, fraction = option.pmip_timestamp) to use attribute access, and update the tests that unpack or tuple() these values (tests/protocols/internet/test_mh_unit.py:2184, :2190).
• Add .. versionchanged:: 1.5.0 on both classes and on docs/source/pcapkit/protocols/internet/mh.rst.
• Correct docs/source/changelog/1.5.0.rst:384-386, which says no NamedTuple remains in the package. TCPSegment (toolkit/pcap.py:45) is one.
• Test the new behaviour, and the dumper output for both classes.