Pytype

New features and updates:
* Support running pytype under Python 3.9. (Does not yet support analyzing 3.9
code; see details in https://github.com/google/pytype/pull/840.)
* Update typeshed pin to commit 869238e from Jan 26.

Bug fixes:
* Do stricter filtering of container_type_mismatch errors.
* Preserve the concrete value of TYPE_CHECKING in method bodies.
* Support using collections.abc.Callable in type stubs.

Bug fixes:
* Fix 'nothing' parameter inference for self in custom generic classes.
* Fix several cases of class type parameters being instantiated incorrectly.
* Fix an if-splitting bug by (mostly) ignoring cycles in the CFG.
* Use match_var_against_type, not match_from_mro, for check_container_types.

New features and updates:
* Replace namedtuples with attrs in our PyTD representation.

Bug fixes:
* Improve error messaging for typing.AnyStr.
* Fully qualify `std::size_t` to avoid platform incompatibilities.
* Add isascii attribute to str, bytes, and bytearray.
* Support isinstance() checks on tuples of typing containers.
* Improve *args and **kwargs forwarding for attrs and dataclasses.
* Don't propagate attribute errors for deleted names.

New features:
* Update typeshed pin to commit 8c20938 from Nov 13.
* Implement basic support for PEP 593.
* Support typeshed's new directory structure.

Bug fixes:
* Support aliases attr.attrs for attr.s, and attr.attrib for attr.ib.
* --check-container-types: fix a crash and improve the error message.

* Switch to a typed_ast-based stub parser.
* Fix matching of NamedType against LiteralType in --protocols mode.
* Fix: super() in a list comprehension needs to look at the enclosing frame.

* Fix some corner cases with unpacking and function args.
* Add ImportError attributes name, path for Python 3.3+.
* Unbreak annotate-ast by adding an empty pytype/ast/__init__.py file.