DFReader, mavutil: support use as context managers

[mgwedd]

What

Adds __enter__/__exit__ to the two base classes that already have a
close() contract:

• DFReader.DFReader — covers DFReader_binary and DFReader_text
• mavutil.mavfile — covers every connection type and log reader
  returned by mavlink_connection()

so callers can scope a reader or connection with a plain with
statement:

with DFReader_binary(path) as dflog:
    ...
# filehandle and mmap released here, even on exception

Why

DFReader_binary.__init__ opens the log file and mmaps its descriptor,
so each reader pins a filehandle until close() is called. Since there
is no context manager support, callers either remember to call close()
manually, wrap the reader in contextlib.closing, or — most commonly —
just leak the handle. ArduPilot's own autotest suite and Replay checker
did the latter for years (ArduPilot/ardupilot#32533, fixed in
ArduPilot/ardupilot#33839. It had to use contextlib.closing because this class can't be used in a with block).

This PR fixes the issue for all subclasses and downstream consumers.

Testing

New tests/test_context_manager.py (runs in the existing pytest suite,
no new fixtures — reuses tests/test.BIN):

• DFReader_binary used in a with block: reads a message, then both
  filehandle and data_map report closed on exit.
• mavlink_connection() on a dataflash log: same via the dispatch path.
• mavlink_connection('udpout:...'): socket is open inside the block,
  closed (fileno == -1) on exit.

All three pass locally on Python 3.13 (macOS); flake8 clean with the
repo config.

No behavior change for existing callers — close() semantics are
untouched, and __exit__ returns None so exceptions propagate.

[khancyr]

LGTM
small changes. exit will call the mavfile override closing method so we are good.
Still work on py3.9
test are simple but OK

[peterbarker]

LGTM

[peterbarker]

Merged, thanks!