bpo-41006: collections imports lazily heap

[vstinner]

The collections module now import lazily the heapq modules in the
Counter.most_common() method to speedup Python startup time.

https://bugs.python.org/issue41006

[vstinner]

This change has no significant impact on most_common() method performance:

$ python3 -m pyperf compare_to master.json lazy_heapq.json 
Benchmark hidden because not significant (1): timeit

I used the commands:

./python -m venv env
env/bin/python -m pip install pyperf
env/bin/python -m pyperf timeit -s 'import collections; counter=collections.Counter("abcdefabc")' 'counter.most_common()'

[vstinner]

2 hours later, not Travis CI build was started :-( I cannot see my PR at https://travis-ci.org/github/python/cpython/pull_requests

I close/reopen the PR to force a new Travis CI job.