Replace S2 timezone handling from pytz to zoneinfo

[Copilot]

S2 modules were still relying on pytz (timezone() / localize()), which is legacy for modern Python timezone handling. This change migrates S2 timezone construction and aware datetime creation to stdlib zoneinfo.

• Scope: S2 timezone migration

  • Replaced pytz imports/usages with zoneinfo.ZoneInfo in:
    • src/flexmeasures_client/s2/__init__.py
    • src/flexmeasures_client/s2/control_types/FRBC/frbc_simple.py
    • src/flexmeasures_client/s2/control_types/FRBC/frbc_tunes.py
    • src/flexmeasures_client/s2/script/websockets_client.py

• Behavioral updates

  • Replaced timezone object creation:
    • pytz.timezone(name) → ZoneInfo(name)
  • Replaced pytz.localize(...) pattern with native aware datetime construction:
    • datetime(..., tzinfo=ZoneInfo(...))
  • Updated FRBCSimple.now() to generate aware datetimes via:
    • datetime.now(self._timezone)

• Focused coverage

  • Added tests/s2/test_timezone.py to assert S2 now() paths produce ZoneInfo-backed datetimes (Handler, FRBCSimple).

from datetime import datetime
from zoneinfo import ZoneInfo

tz = ZoneInfo("Europe/Amsterdam")
now = datetime.now(tz)
valid_from = datetime(2026, 1, 15, 20, tzinfo=tz)

[nhoening]

I would like to leave it to @Flix6x to give this a final look.
I researched the only thing that gave me thought (see comment below).