Fixed also testing the "always" warning filter.

Add missing warning filter to test_exec_filename().

More consistent with how other parts of Python find the filename (e.g. tracebacks and pdb).

Fix test_warnings.test_module_globals() when python3 is run with
-Werror.

Don't crash on warnings.warn_explicit() if module_globals is not a dict.

- primary change is to add a new default filter entry for
  'default::DeprecationWarning:__main__'
- secondary change is an internal one to cope with plain
  strings in the warning module's internal filter list
  (this avoids the need to create a compiled regex object
  early on during interpreter startup)
- assorted documentation updates, including many more
  examples of configuring the warnings settings
- additional tests to ensure that both the pure Python and
  the C accelerated warnings modules have the expected
  default configuration

Rather than supporting dev mode directly in the warnings module, this
instead adjusts the initialisation code to add an extra 'default'
entry to sys.warnoptions when dev mode is enabled.

This ensures that dev mode behaves *exactly* as if `-Wdefault` had
been passed on the command line, including in the way it interacts
with `sys.warnoptions`, and with other command line flags like `-bb`.

Fix also bpo-20361: have -b & -bb options take precedence over any
other warnings options.

Patch written by Nick Coghlan, with minor modifications of Victor Stinner.

* Add most_recent_first parameter to tracemalloc.Traceback.format to allow
   reversing the order of the frames in the output
* Reversed default sorting of tracemalloc.Traceback frames
* Allowed negative limit, truncating from the other side.

The warnings module doesn't leak memory anymore in the hidden
warnings registry for the "ignore" action of warnings filters.

The warn_explicit() function doesn't add the warning key to the
registry anymore for the "ignore" action.