core: import some stuff from collections.abc (deprecation update)

Since Python 3.3, some imports have moved from collections to
collections.abc but were kept for compatibility, though compatibility
will be dropped in Python 3.10.
We will keep compatibility ourselves for some time again and thus support
both imports.
7 jobs for master in 15 minutes and 55 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #29933
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  Test
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pyflakes:3

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pyflakes-strict:3

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unittests:3

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failed #29938
allowed to fail
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  Deploy
passed #29939
doc:3

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Name Stage Failure
failed
unittests-modules:3 Test
        </head>
<body>
<h1>Error: 400 Bad Request</h1>
<p>Sorry, the requested URL <tt>'https://ci.weboob.org/api/v1/modules'</tt>
caused an error:</p>
<pre>Invalid JSON payload.</pre>
</body>
</html>
ERROR: Job failed: exit code 1