Issue #18426: Fix NULL pointer dereference in C extension import when
PyModule_GetDef() returns an error.
Issue #18426: improve exception message. Courtesy of Amaury
Issue #18336. Fix a link to StreamReader's read() method.
Fix reference leaks introduced by the patch for issue #5308.
#17987: properly document support.captured_xxx.

Patch by Dmi Baranov.
Issue #18101: Tcl.split() now process strings nested in a tuple as it
do with byte strings.

Added tests for Tcl.split() and Tcl.splitline().
Issue #18085: Add missed const modifier for some entries in refcounts.dat.
Issue #17872: Fix a segfault in marshal.load() when input stream returns
more bytes than requested.
#18044: Fix parsing of encoded words of the form =?utf8?q?=XX...?=

The problem was I was only checking for decimal digits after the third '?',
not for *hex* digits :(.

This changeset also fixes a couple of comment typos, deletes an unused
function relating to encoded word parsing, and removed an invalid
'if' test from the folding function that was revealed by the tests
written to validate this issue.