Patch by sbt.

Call also directly unicode_resize() in unicodeobject.c

Fix also PyUnicode_Resize() doc

Patch by Filip Gruszczyński.

Undocument the function.

Make also decode_utf8_errors() as private (static).

 * Rename unicode_resizable() to unicode_modifiable()
 * Rename _PyUnicode_Dirty() to unicode_check_modifiable() to make it clear
   that the function is private
 * Inline PyUnicode_Concat() and unicode_append_inplace() in PyUnicode_Append()
   to simplify the code
 * unicode_modifiable() return 0 if the hash has been computed or if the string
   is not an exact unicode string
 * Remove _PyUnicode_DIRTY(): no need to reset the hash anymore, because if the
   hash has already been computed, you cannot modify a string inplace anymore
 * PyUnicode_Concat() checks for integer overflow

It has no sense to check if a not ready string is ASCII or not.

If maxchar_new == 0 and self is a unicode subtype, return u instead of duplicating u.

_PyUnicode_CheckConsistency() also checks string content.

 * Remove micro-optimization from PyUnicode_FromStringAndSize():
   PyUnicode_DecodeUTF8Stateful() has already these optimizations (for size=0
   and one ascii char).
 * Rename utf8_max_char_size_and_char_count() to utf8_scanner(), and remove an
   useless variable

 * Create decode_utf8_errors()
 * Reuse unicode_fromascii()
 * decode_utf8_errors() doesn't refit at the beginning
 * Remove refit_partial_string(), use unicode_adjust_maxchar() instead

Issue #13248: turn 3.2's PendingDeprecationWarning into 3.3's DeprecationWarning (cgi, importlib, nntplib, smtpd).

Test the following functions:

 * codecs.raw_unicode_escape_decode()
 * PyUnicode_FromWideChar()
 * PyUnicode_FromUnicode()
 * "unicode_internal" and "unicode_escape" decoders