* Correctly declare the parameter annotations as `BinaryIO`, because in fact it works only with byte-based buffers.

* `sys.stdin.read()` is always a string, so remove the Python2-specific check.
    * Correct the patch in the test which incorrectly sets `sys.stdin` to a bytes buffer which it never is.

* `sys.stdout.buffer` always exists, so remove the Python2-specific fallback.
    * Correct the patch in the test which incorrectly sets `sys.stdout` to a bytes buffer which it never is. And fix a matching mistake because this necessitated `stdout.read().decode()`.

Switch to a different action (cpina/github-action-push-to-another-repository) which has worked in testing (see https://github.com/waylan/test-repo-source).

Also updated the version of many actions in all workflows to eliminate various warnings we have been getting of late.

There are no other references to `content_indention`, and it seems that there were no problems with this only because the other line `self.content_indent = indent` kicks in in all relevant situations.

If there is an empty `<pre><code></code></pre>` inserted by another extension, there can be an exception. The added test case was crashing before this change.

If an inlineprocessor returns an AtomicString (even though that is pointless, a plain string is atomic in that context), there can be an exception in 2 separate places. The added test case was crashing before this change.

The update in 5cbabe1 didn't provide for a .nojekyll file, which is necessary when publishing to GitHub Pages to avoid Jekyll messing with our statically generated documentation.

Also added a user input to the manual deploy script which allows deploying from any past reference (tag or commit).

Also fix a typo (`path=>paths`).

Fixes #1416.

As with most implementations, we now pass through bogus comments (as
defined by the HTML Spec) unaltered except that they are HTML escaped.
This deviates from the reference implementation which completely ignores
them. As the reference implementation seems to not have even contemplated
their existence, it is not being used as a reference in this instance.
Fixes #1425.