Makes `import functools` about 1.5 ms faster.

* heapq is used only when Counter.most_common(n).
* types and waakref is used only for singledispatch.

master:

import time:   - _functools 76 us (self 76 us)
import time:       - _operator 110 us (self 110 us)
import time:     - operator 469 us (self 359 us)
import time:     - keyword 174 us (self 174 us)
import time:       - _heapq 189 us (self 189 us)
import time:     - heapq 400 us (self 211 us)
import time:     - itertools 137 us (self 137 us)
import time:     - reprlib 231 us (self 231 us)
import time:     - _collections 142 us (self 142 us)
import time:   - collections 3006 us (self 1456 us)
import time:     - collections.abc 191 us (self 191 us)
import time:   - types 518 us (self 327 us)
import time:   - weakref 577 us (self 577 us)
import time: - functools 4903 us (self 729 us)

patched:

import time:   - _functools 74 us (self 74 us)
import time:       - _operator 100 us (self 100 us)
import time:     - operator 460 us (self 361 us)
import time:     - keyword 174 us (self 174 us)
import time:     - itertools 139 us (self 139 us)
import time:     - reprlib 226 us (self 226 us)
import time:     - _collections 85 us (self 85 us)
import time:   - collections 2593 us (self 1512 us)
import time: - functools 3369 us (self 703 us)

Initially my patch for issue30152 included the change in collections. But it was removed on Raymond's request.

Go ahead.

An option for deferred imports would be to leave a comment at the top where the import would otherwise be.  Example:

# import types, weakref  # Deferred to single_dispatch()

This accomplishes the PEP8 purpose of making all dependencies of the module visible at the top.

New changeset 9811e80fd0ed9d74c76a66f1dd4e4b8afa9e8f53 by INADA Naoki in branch 'master':
bpo-31581: Reduce the number of imports for functools (GH-3757)
https://github.com/python/cpython/commit/9811e80fd0ed9d74c76a66f1dd4e4b8afa9e8f53