Add chess.pgn.Game.set_time_control()

[ybai08]

Closes #1033.

Adds Game.set_time_control(time_control), the setter counterpart of Game.time_control(). It writes the TimeControl header in the PGN spec format (section 9.6.1), so that parse_time_control() reads back an equal TimeControl.

• Parts are joined with :. A part is written as moves/seconds, or as plain seconds for sudden death, followed by +increment or +delayd where set. Fractional increments and delays are written with up to 3 decimals, the same way set_clock() writes them (180+0.5, 300+2).
• A TimeControl without parts is written as - when its type is UNLIMITED, and as ? otherwise.
• ValueError is raised, and the header is left unchanged, for values that parse_time_control() would reject or cannot express: a sudden-death part before the last part, or a part with both an increment and a delay.

Game.time_control and Game.set_time_control are also added to the Game members listed in docs/pgn.rst.

The new test PgnTestCase.test_set_time_control round-trips headers (40/7200:20/3600:900+30, 300+2, 180+0.5, 5400+30d, 1800, -, ?) and checks the error cases.

Checks run locally:

• python test.py on Python 3.14 and 3.9: 297 tests OK (22 skipped, engine binaries not installed). The new test fails on master with AttributeError: 'Game' object has no attribute 'set_time_control'.
• python -m mypy --strict chess: no issues.
• python -m pyright chess/pgn.py: 0 errors (one existing warning about typing_extensions).
• python -m doctest README.rst: only the 3 Stockfish examples fail, because no stockfish binary is installed.

Not run: Windows CI and the perft workflow, which this change does not touch.