This adds:
- Add ASYNC/AWAIT tokens back to Grammar and regenerate
- Recognize async/await keywords conditionally if feature_version < 7
- Reject await expressions if feature_version < 5
- Docs for ASYNC/AWAIT tokens and for ast.parse(..., feature_version=N)

This is everything currently in typeshed except await expressions (but
it does reject async functions etc.):

    - Reject async functions and async for/with in minor versions < 5

    - Reject async comprehensions in minor versions < 6

    - Reject underscores in numeric literals in minor versions < 6

    - Reject variable annotations in minor versions < 6

    - Reject `X @= Y` in minor versions < 5

Visible behavior:

- Issue error for `X @ Y` (matrix multiply) if c_feature_version < 5

- Add optional feature_version kw arg to ast.parse() (default -1 which
  implies PY_MINOR_VERSION)

- Add feature_version: int = -1 to compile() (via Argument Clinic);
  this sets cf_feature_version to the given value if >= 0, else
  defaults to PY_MINOR_VERSION

Implementation:

- Add PyAST_obj2mod_ex(): like PyAST_obj2mod() but with feature_version arg;
  the latter calls the former with PY_MINOR_VERSION

- Add cf_feature_version to PyCompilerFlags structure;
  initialized to PY_MINOR_VERSION everywhere

- Add c_feature_version to struct compiling; initialize from
  cf_feature_version

- Add 'c' argument to get_operator()

- In builtin eval() and exec(), default to PY_MINOR_VERSION

TODO:

- Put version-dependent ASYNC/AWAIT keyword scanning back

- Reject async functions, await expressions, and async for/with in minor versions < 5

- Reject async comprehensions in minor versions < 6

- Reject underscores in numeric literals in minor versions < 6

- Reject variable annotations in minor versions < 6

- Reject `X @= Y` in minor versions < 5

NamedStore has been replaced with Store. The difference between
NamedStore and Store is handled when precess the NamedExpr node
one level upper.

* Move 'allocator' and 'dev_mode' fields from _PyCoreConfig
  to _PyPreConfig.
* Fix InitConfigTests of test_embed: dev_mode sets allocator to
  "debug", add a new tests for env vars with dev mode enabled.

* Move following fields from _PyCoreConfig to _PyPreConfig:

  * coerce_c_locale
  * coerce_c_locale_warn
  * legacy_windows_stdio
  * utf8_mode

* _PyPreConfig_ReadFromArgv() is now responsible to choose the
  filesystem encoding
* _PyPreConfig_Write() now sets the LC_CTYPE locale

The new function is now responsible to parse -E and -I command line
arguments.

* Add _PyPreConfig structure
* Move 'ignored' and 'use_environment' fields from _PyCoreConfig
  to _PyPreConfig
* Add a new "_PyPreConfig preconfig;" field to _PyCoreConfig