typing.ContextManager.

Exceptions raised during converting argument of correct type are no longer
overridded with TypeError. Some error messages are now more detailed.

ByteArrayAsStringTest.fixtype() was converting test data to bytes, not byte-
array, therefore many of the test cases inherited in this class were not
actually being run on the bytearray type.

The tests in buffer_tests.py were redundant with methods in string_tests
.MixinStrUnicodeUserStringTest and string_tests.CommonTest. These methods are
now moved into string_tests.BaseTest, where they will also get run for bytes
and bytearray.

This change also moves test_additional_split(), test_additional_rsplit(), and
test_strip() from CommonTest to BaseTest, meaning these tests are now run for
bytes and bytearray. I plan to eliminate redundancies with existing tests in
test_bytes.py soon.

This syncs to rev 7b43ada77821d23e55e3a4b35f6055a59b9e1ad7 there.

Summary:

- Add typing.DefaultDict (as a generic variant of collections.defaultdict).

- Use collections.Reversible if it exists (only relevant for Python 3.6).

- Revamped generic class behavior to conform to updated PEP 484.

- Improve speed of Generic.__new__.

- Make sure __init__ is called for new Generic instances. Fix issue #26391.

- Refactor async support to be compatible with 3.2, 3.3, 3.4.

- Remove 'io' and 're' from __all__ (they still exist, just not
  included by "import *"). Fix issue #26234.

- Change @overload -- you can now use it outside stubs (you still
  cannot call the decorated function though).

* Group functions into six new subsections
* Document the underlying Readline function or variable accessed
* get_history_length() returns the history file limit
* clear_history() is conditionally compiled in
* Clarify zero and one bases for history item indexes
* parse_and_bind() uses its argument directly as an init line
* Change "command line" to "line buffer" for consistency
* read_init_file() also executes the file
* read_history_file() replaces the previous history
* write_history_file() overwrites any existing file
* Differentiate history file lines from history list items, which could be
  multi-line
* Add more information about completion, also addressing Issue #10796
* libedit (Editline) may be used on any platform; detection is OS X specific

Patch by Robert Bachmann and SilentGhost.

Patch by Robert Bachmann and SilentGhost.

Patch by Ashley Anderson.

Patch by Ashley Anderson.

Also fix links to the “tzinfo” class and attributes.