Fix "Uncaught TypeError: Cannot read property 'severity' of undefined"

Closes #3958

Issue #3973

@ as __matmul__ operator only on Python 3,

Add tests for Python mode for this use case.

Closes #3957

Issue #3974

Even if we don't use promise internally, check for the existence of
`.then` attributes on the return results of a hinting function, and if
so assume it is a promise.

Internally also convert all hinters to use the asynchronous interface
for consistency and simplification of code paths.

Closes #3971

`u` prefix are valid (again) since 3.3, and `f` prefix are now allowed
starting at 3.6 (which is not released yet though).

Closes #3972

Issue #3956

There were two issues:

1. Github flavored markdown calls Objective C `objective-c`
2. The gfm.js mode didn't include `-` in its fenced code block regex

Additionally, test/index.html did not import the mode/meta.js file,
which prevented mode lookup by name from working in unit tests.

Issue #3941

Issue #3932

BUCK, BUILD, *.BUILD and *.bzl files are build files for the three build
systems mentioned, and they are all valid Python files.

5.13.4 was a trivial release restoring a LICENSE file.

Issue #3938

Closes #3934

Closes #3922

Closes #3920