Freeze all the objects tracked by gc - move them to a permanent generation
and ignore all the future collections. This can be used before a POSIX
fork() call to make the gc copy-on-write friendly or to speed up collection.

* Rewrite win_perf_counter() to only use integers internally.
* Add _PyTime_MulDiv() which compute "ticks * mul / div"
  in two parts (int part and remaining) to prevent integer overflow.
* Clock frequency is checked at initialization for integer overflow.
* Enhance also pymonotonic() to reduce the precision loss on macOS
  (mach_absolute_time() clock).

Patch by Pablo.

Follow-up to Victor's patch that enabled autotest to run in IDLE's Shell.

This sentence is removed while backporting to 3.6 branch.
See https://github.com/python/cpython/pull/3982#discussion_r144555768

The _PyTime API handles detects overflow and is well tested.

Document that the signal will only be sent once if internal is equal
to zero.

test_load_source() now replaces the current __name__ module with a
temporary module to prevent side effects.

LoadLibrary("SHELL32") is not vulnerable to DLL hijacking.

When regrtest in run inside IDLE, sys.stdout and sys.stderr are not
TextIOWrapper objects and have no file descriptor associated:
sys.stderr.fileno() raises io.UnsupportedOperation.

Disable faulthandler and don't replace sys.stdout in that case.

Pattern `[a-z]` with `IGNORECASE` flag can match to some non-ASCII characters.

Straightforward solution for this is using `IGNORECASE | ASCII` flag.
But users may subclass `Template` and override only `idpattern`. So we want to
avoid changing `Template.flags`.

So this commit uses local flag `-i` for `idpattern` and change `[a-z]` to `[a-zA-Z]`.

Reported by John Gamboa on docs@p.o at
https://mail.python.org/pipermail/docs/2017-June/031942.html

time.clock() and time.perf_counter() now use again C double
internally.

Remove also _PyTime_GetWinPerfCounterWithInfo(): use
_PyTime_GetPerfCounterDoubleWithInfo() instead on Windows.

* Move _PyTime_overflow() at the top
* Move assertion on numerator into _PyTime_ObjectToDenominator()
* PEP 7: add { ... } to if blocks

* Separated functions and constants descriptions in sections.
* Added a note about the limitations of timezone constants.
* Removed redundant lists from the module docstring.

Change the code example from using `get_history_length` to `get_current_history_length`.

bpo-31728: Prevent crashes in _elementtree due to unsafe cleanup of Element.text and Element.tail (#3924)

* Add _PyTime_GetPerfCounter()
* Use _PyTime_GetPerfCounter() for -X importtime

bpo-31701: On Windows, faulthandler.enable() now ignores MSC and COM
exceptions.

bpo-28280: Make PyMapping_Keys(), PyMapping_Values() and PyMapping_Items() always return a list (#3840)