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API 4.8 support

Pure API changes

• Bot.send_poll: explanation, explanation_parse_mode,
• Poll.{explanation, explanation_entities, open_period, close_date}
• Bot.send_dice: emoji (for whatever reason, sending a 'darts' is considered a die now …
• Dice.emoji

Convenience stuff

• Poll.parse_explanation_entity/ies
• Filters.dice.dice and Filters.dice.darts for filtering only the corresponding emojis. Allow the Filters.dice.*([1, 2]) syntax for both