* Set but unused variable in Parser/pgen.c in non-debug builds. Patch by
Christian Heimes.
* Unused static function in Modules/readline.c. Patch by Georg Brandl.
* main_window unused in Modules/tkappinit.c. Patch by Gregory P. Smith.
* Dead assignment in Modules/_ctypes/cfield.c. Extracted from patch by Brett
Cannon.
* Expression result unused in PyObject_INIT macro expansions. Based on
patches by Christian Heimes.
* Load expat_config.h and therefore pyconfig.h before C stdlib headers are
loaded. This silences pre-processor warnings including '_POSIX_C_SOURCE
redefined'. Extracted from patch by Christian Heimes.
Backported issues #19619: encode() and decode() methods and constructors
of str, unicode and bytearray classes now emit deprecation warning for known
non-text encodings when Python is ran with the -3 option.
Backported issues #20404: io.TextIOWrapper (and hence io.open()) now uses the
internal codec marking system added to emit deprecation warning for known non-text
encodings at stream construction time when Python is ran with the -3 option.
Remove an unused early-out test from the critical path for
dict and set lookups.
When the strings already have matching lengths and hashes,
there is no additional information gained by checking the first
characters (the probability of a mismatch is already known to
be less than 1 in 2**64).
This fixes two places where 'int' was used to represent
the size of strings, instead of 'Py_ssize_t'.
(The issue is not present in the corresponding code in the 3.x branches)
Fixes#18427
