* bpo-40394 - difflib.SequenceMatched.find_longest_match default args

Added default args to find_longest_match, as well as related tests.

* "Return true/false" is replaced with "Return ``True``/``False``"
  if the function actually returns a bool.
* Fixed formatting of some True and False literals (now in monospace).
* Replaced "True/False" with "true/false" if it can be not only bool.
* Replaced some 1/0 with True/False if it corresponds the code.
* "Returns <bool>" is replaced with "Return <bool>".

This reverts commit 78de01198b047347abc5e458851bb12c48429e24.

* Prevent low-grade poplib REDOS (CVE-2018-1060)

The regex to test a mail server's timestamp is susceptible to
catastrophic backtracking on long evil responses from the server.

Happily, the maximum length of malicious inputs is 2K thanks
to a limit introduced in the fix for CVE-2013-1752.

A 2KB evil response from the mail server would result in small slowdowns
(milliseconds vs. microseconds) accumulated over many apop calls.
This is a potential DOS vector via accumulated slowdowns.

Replace it with a similar non-vulnerable regex.

The new regex is RFC compliant.
The old regex was non-compliant in edge cases.

* Prevent difflib REDOS (CVE-2018-1061)

The default regex for IS_LINE_JUNK is susceptible to
catastrophic backtracking.
This is a potential DOS vector.

Replace it with an equivalent non-vulnerable regex.

Also introduce unit and REDOS tests for difflib.

Co-authored-by: Tim Peters <tim.peters@gmail.com>
Co-authored-by: Christian Heimes <christian@python.org>

This hides unwanted implementation details from tracebacks.

And most of the tools.

Patch by Emanual Barry, reviewed by me, Serhiy Storchaka, and
Martin Panter.