Automerge-Triggered-By: @brettcannon

* bpo-39336: Allow setattr to fail on modules which aren't assignable

When attaching a child module to a package if the object in sys.modules raises an AttributeError (e.g. because it is immutable) it causes the whole import to fail.  This now allows immutable packages to exist and an ImportWarning is reported and the AttributeError exception is ignored.

Relative imports use resolve_name to get the absolute target name,
which first seeks the current module's absolute package name from the globals:
If __package__ (and __spec__.parent) are missing then
import uses __name__, truncating the last segment if
the module is a submodule rather than a package __init__.py
(which it guesses from whether __path__ is defined).

The __name__ attempt should fail if there is no parent package (top level modules),
if __name__ is '__main__' (-m entry points), or both (scripts).
That is, if both __name__ has no subcomponents and the module does not seem
to be a package __init__ module then import should fail.

bpo-15386, bpo-37473: test_import, regrtest and libregrtest no longer
import importlib as soon as possible, as the first import, "to test
bpo-15386".

It is tested by test_import.test_there_can_be_only_one().

Sort test_import imports.

This commit contains the implementation of PEP570: Python positional-only parameters.

* Update Grammar/Grammar with new typedarglist and varargslist

* Regenerate grammar files

* Update and regenerate AST related files

* Update code object

* Update marshal.c

* Update compiler and symtable

* Regenerate importlib files

* Update callable objects

* Implement positional-only args logic in ceval.c

* Regenerate frozen data

* Update standard library to account for positional-only args

* Add test file for positional-only args

* Update other test files to account for positional-only args

* Add News entry

* Update inspect module and related tests