Contains security fixes for CVE-2019-20907, CVE-2020-15801, and BPO-39603.

* bpo-40826: Fix GIL usage in PyOS_Readline() (GH-20579)

Fix GIL usage in PyOS_Readline(): lock the GIL to set an exception.

Pass tstate to my_fgets() and _PyOS_WindowsConsoleReadline(). Cleanup
these functions.

(cherry picked from commit c353764fd564e401cf47a5d9efab18c72c60014e)

* bpo-40826: Add _PyOS_InterruptOccurred(tstate) function (GH-20599)

my_fgets() now calls _PyOS_InterruptOccurred(tstate) to check for
pending signals, rather calling PyOS_InterruptOccurred().

my_fgets() is called with the GIL released, whereas
PyOS_InterruptOccurred() must be called with the GIL held.

test_repl: use text=True and avoid SuppressCrashReport in
test_multiline_string_parsing().

Fix my_fgets() on Windows: fgets(fp) does crash if fileno(fp) is closed.

(cherry picked from commit fa7ab6aa0f9a4f695e5525db5a113cd21fa93787)

The constant values of future flags in the __future__ module
is updated in order to prevent collision with compiler flags.
Previously PyCF_ALLOW_TOP_LEVEL_AWAIT was clashing
with CO_FUTURE_DIVISION..
(cherry picked from commit 4454057269b995341b04d13f0bf97f96080f27d0)

Co-authored-by: Batuhan Taşkaya <batuhanosmantaskaya@gmail.com>