Pytype

* Fix a bug in attaching function type comments to opcodes.
* Use `py -x.y` instead of `pythonx.y` for windows.

* Fix a bug involving multiple super calls.
* Support tuple slicing.
* Make all tests pass in Python 3.8.
* Respect all typing re-exports in the pyi parser.
* Create C++ metrics classes for tracking typegraph metrics.
* Update attrs pytd files to support version 20.2.0.

* Fix a parser bug when importing and re-exporting a submodule.
* Call `python -m ninja` instead of `ninja`.
* Don't crash when processing a pyi containing __all__.
* Convert Bindings() to a list in cfg.py.

* Drop the --python_exe option.
* Have check_py and generate_pyi accept a source string instead of a filename.
* Pass around an options.open_function that is used in place of builtins.open.
* Fix some Python 3.8 bugs.
* Fix python-incompatible syntax in pytd files.
* Print 'leaving directory' after ninja command.
* Support sys.version_info tuple checks that do not use slicing.

* Let multiple PyTDFunction signatures match when *args/**kwargs is present.

* Allow ` type: ignore` in more places in pyi files.
* Support TypedDict function-based syntax in pyi files.
* Add more developer documentation.