Dataclasses: Fix example on 30.6.8, add method should receive a list rather than an integer. (GH-8038)

Change example function to append rather than add lists.

The ValueError message ends with a period.

Many users won't realise the implications the OpenSSL 1.0.2
minimum version requirement has when it comes to Ubuntu
14.04 and Debian 8, so expand on that in the platform support
section.

Also explicitly note the non-ASCII-based locale requirement for
full Unicode text handling support on non-Windows systems.

Contributed by Bradley Laney.

Follow up from https://github.com/python/peps/pull/712#issuecomment-403591108:

"people using the repo without using GitHub still ought to be reminded of the COC" - GvR

* When doing getattr lookups with a default of "None", it now
  uses an "is" comparison against None which is more correct
* Removed outdated code

Patch by Brandon Rhodes.

Initial patch by Joshua Jay Herman.

Fixed also testing the "always" warning filter.

Include a more easily understood example for nullcontext

This will prevent emitting a resource warning when the execution was
interrupted by Ctrl-C between calling open() and entering a 'with' block
in "with open()".

webbrowser._synthesize() called webbrowser.register() with
outdated signature.
Co-Authored-By: John Still <john@jmsdvl.com>

Make the change where discussing the CPython implementation of lists and dicts.

The 'output formatting' section of the tutorial talks a lot about manual formatting with things like .rjust() and .zfill(), with only a passing reference to 3.6's new f-strings.

This doesn't drop all of the old material, but it does rearrange the topics into a more modern order: f-strings first, discussing formatting specifiers a bit; then calling .format(); finally manual formatting with .ljust().

"can be used do declare" → "can be used to declare"

* Always return bytes from _HackedGetData.get_data().

Ensure the imp.load_source shim always returns bytes by reopening the file in
binary mode if needed. Hash-based pycs have to receive the source code in bytes.

It's tempting to change imp.get_suffixes() to always return 'rb' as a mode, but
that breaks some stdlib tests and likely 3rdparty code, too.

Stop using key=lambda.  This behavior is same to C version encoder.