readme: move the support channel on libera

This lingered a bit but the Freenode situation is becoming clownier and clownier
by the hour.

--HG--
branch : stable
8 jobs for topic/stable/foolish-fly in 4 minutes and 36 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed checks-py2 #206623

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passed checks-py3 #206624

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passed doc #206629

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manual pytype #206622
allowed to fail manual
passed tests-py2-cext #206625

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passed tests-py2-pure #206626

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passed tests-py3-cext #206627

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passed tests-py3-pure #206628

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