Fix test_dbm_gnu.test_reorganize() on macOS with gdbm 1.15: add a
larger value to make sure that the file size changes.

bpo-33671
* use memoryview() with size == file size on Windows, see https://github.com/python/cpython/pull/7160#discussion_r195405230
* release intermediate (sliced) memoryview immediately
* replace "OSX" occurrences with "macOS"
* add some unittests for copyfileobj()

with debuglevel=1 only the header keys got printed. With
this change the header values get printed as well and the single
header entries get '\n' as a separator.

Using gdbm 1.15, creating a database creates a file of 16 MiB. Adding
a small entry and then modifying the small entry doesn't change the
file size. Modify test_dbm_gnu to be less strict: allow that the file
size doesn't change.

This may help to investigate bpo-33901.

Part 2 of 3.  Continues PR #7689, changeset ee5ef309.
Edit and add tests for 18 modules, help_about to replace and run.

Make it the same as when one runs 'python'.

* Test exists(), lexists(), isdir(), isfile(), islink(), ismount()
  with bytes paths.
* Remove unneeded silencing DeprecationWarning for ismount() with
  bytes path.
* Test common functions with unencodable and undecodable paths.
* Minor clean up and refactoring.

In some development setups it is inconvenient or impossible to write bytecode
caches to the code tree, but the bytecode caches are still useful. The
PYTHONPYCACHEPREFIX environment variable allows specifying an alternate
location for cached bytecode files, within which a directory tree mirroring the code
tree will be created. This cache tree is then used (for both reading and writing)
instead of the local `__pycache__` subdirectory within each source directory.

Exposed at runtime as sys.pycache_prefix (defaulting to None), and can
be set from the CLI as "-X pycache_prefix=path".

Patch by Carl Meyer.

Mention that it can be triggered by triple quotes and after specifying decorators.

Update the the signature in the code example to make `_cache` a keyword-only parameter.

regrtest: Add warning when using less than 3 warmup runs like -R 1:3.

Create a template for minimally testing a tkinter-using module by importing it and instantiating its class(es).  Add a test file for all non-startup IDLE modules.  Edit existing files and update coverage.  This is part 1 of 3, covering the 21 autocomplete to help modules and touching 33 idlelib files.

Fix "LC_ALL=C python3.7 -V": reset properly the command line parser
when the encoding changes after reading the Python configuration.

Fix pymain_read_conf(): use memset(0) to reset properly cmdline.

Revert "bpo-32962: Fix test_gdb failure in debug build with -mcet -fcf-protection -O0 (GH-6754)" (#7723)

This reverts commit 9b7c74ca.

'expresson list' refers to the grammar term 'expression_list' in the subscription production.

When Python is built with the intel control-flow protection flags,
-mcet -fcf-protection, gdb is not able to read the stack without
actually jumping inside the function. This means an extra
'next' command is required to make the $pc (program counter)
enter the function and make the stack of the function exposed to gdb.

Marked files are collapsed by default in Github pull request.
https://github.com/github/linguist#generated-code

Hangul composition check boundaries are wrong for the second character
([0x1161, 0x1176) instead of [0x1161, 0x1176]) and third character ((0x11A7, 0x11C3)
instead of [0x11A7, 0x11C3]).

The existing doc had a number of info gaps and was a little hard to use.  This patch provides several quick-reference tables as well as examples.

python-gdb now catchs UnicodeDecodeError exceptions when calling
string().

python-gdb now catchs ValueError on read_var(): when Python has no
debug symbols for example.

* Enhance also format_duration(): work on integers and rounds towards
  +infinity (math.ceil).
* Write unit tests on format_duration()

Testing strategy: whitebox.

All the subinterpreter tests were disabled in gh-7513. This commit re-enables them, but leaves one bad test disabled. The test is partly causing problems because it makes assumptions about the availability of a high-level interpreters module (see PEP 554). So I'm disabling the test until such a high-level module is available.

string.Formatter auto-numbering feature was added in 3.4 and there
is no versionchanged note in its documentation, making the documentation
ambiguous about which version the feature is available.