Bug#1000315: compreffor: FTBFS with Python 3.10

[Stefano Rivera <stefanor@debian.org>]

Source: compreffor
Version: 0.5.1-1
Severity: serious
Tags: ftbfs

https://buildd.debian.org/status/fetch.php?pkg=compreffor&arch=amd64&ver=0.5.1-1%2Bb1&stamp=1637495198&raw=0

Tests fail with:
==================================== ERRORS ====================================
_ ERROR collecting .pybuild/cpython3_3.10_compreffor/build/compreffor/test/dummy.py _
compreffor/test/dummy.py:18: in <module>
    class DummyGlyphSet(collections.MutableMapping):
E   AttributeError: module 'collections' has no attribute 'MutableMapping'
_ ERROR collecting .pybuild/cpython3_3.10_compreffor/build/compreffor/test/pyCompressor_test.py _
compreffor/test/pyCompressor_test.py:19: in <module>
    from compreffor.test.dummy import DummyGlyphSet
compreffor/test/dummy.py:18: in <module>
    class DummyGlyphSet(collections.MutableMapping):
E   AttributeError: module 'collections' has no attribute 'MutableMapping'
_ ERROR collecting .pybuild/cpython3_3.10_compreffor/build/compreffor/test/pyCompressor_test.py _
compreffor/test/pyCompressor_test.py:19: in <module>
    from compreffor.test.dummy import DummyGlyphSet
compreffor/test/dummy.py:18: in <module>
    class DummyGlyphSet(collections.MutableMapping):
E   AttributeError: module 'collections' has no attribute 'MutableMapping'

The fix should be as simple as:
https://github.com/googlefonts/compreffor/commit/1ddd8416cd2e998d783def2ba392cea9c9fce346

I'll file an NMU momentarily.

SR