v1.7.5 - upgrade to TZDB 2021c; fix edge case bug when Transition occurs at ZoneEra transition

• 1.7.5 (2021-10-06, TZDB 2021c)
  • Bug Fix: Update ExtendedZoneProcessor.h to implement better
    detection of Transitions that occur at the exact same time as the switch
    to a different ZoneEra.
    • They are now considered to happen at the same time if any of the 'w'
      time, 's' time, or 'u' time are equal.
    • The behavior of ExtendedZoneProcessor.h should now be identical
      to zone_processor.py in the AceTimePython library.
    • Seems to affect only Europe/Lisbon in 1992, which is not a part of
      the predefined zonedb or zonedbx database, which normally include
      only 2000 until 2050.
  • Testing
    • Create AceTimePython
      library extracted from the previously split
      AceTimeTools project.
    • Update ace_time/testing/ExtendedTransitionTest.h to validate
      the exact equality between the observed maximum buffer size of
      TransitionStorage as observed by ExtendedZoneProcessor and the
      buffer size calculated by AceTimeTools using zone_processor.py of
      AceTimePython library.
    • Create examples/DebugZoneProcessor for debugging the internal logic
      of ExtendedZoneProcessor.
  • Tool Chain
    • Upgrade ESP8266 Core from 2.7.4 to 3.0.2.
      • Flash consumption increases by 3-5 kB across the boad.
    • Upgrade Arduino CLI from 0.19.1 to 0.19.2.
    • Upgrade Arduino IDE from 1.8.13 to 1.8.16.
    • Upgrade Teensyduino from 1.54 to 1.55.
    • Upgrade SparkFun SAMD Core from 1.8.3 to 1.8.4.
  • TZDB Upgrade TZDB from 2021a to 2021c.
    • TZDB 2021b is skipped because of controversial changes which were
      reverted in 2021c.
    • 2021c announcement:
      https://mm.icann.org/pipermail/tz-announce/2021-October/000067.html
    • 2021b announcement:
      https://mm.icann.org/pipermail/tz-announce/2021-September/000066.html
    • Jordan now starts DST on February's last Thursday.
    • Samoa no longer observes DST.
    • 10 Zones from 2021a were converted to Links:
      • Africa/Accra -> Africa/Abidjan
      • America/Atikokan -> America/Panama
      • America/Blanc-Sablon -> America/Puerto_Rico
      • America/Creston -> America/Phoenix
      • America/Curacao -> America/Puerto_Rico
      • America/Nassau -> America/Toronto
      • America/Port_of_Spain -> America/Puerto_Rico
      • Antarctica/DumontDUrville -> Pacific/Port_Moresby
      • Antarctica/Syowa -> Asia/Riyadh
      • Pacific/Enderbury -> Pacific/Kanton
    • 1 new Zone was created:
      • Pacific/Kanton
    • BasicZoneManager now supports 258 Zones and 193 Links using the
      zonedb database
    • ExtendedZoneManager now supports 377 Zones and 217 Links using the
      zonedbx database