performed by running "pyupgrade --py36-plus" and committing the results

All changes were created by running

"pyupgrade --py3-only --keep-percent-format"

and committing the results. I have not touched string formatting for
now.

- use set literals
- simplify .format() parameter naming
- remove __future__
- remove default "r" mode for open()
- use OSError rather than compatibility aliases
- remove stray parentheses in function(generator) scopes

glob.glob() is not sorted, despite using shell-style wildcards, and the
documentation does not mention this: https://bugs.python.org/issue21748

Recently, it does start mentioning "Whether or not the results are
sorted depends on the file system." which does not really get to the
heart of the matter...

This is causing fuzz too.

The current way this works is chaos since the tool might return files in
any order and thus shuffle around the order of embedded files. This
results in big diffs that cannot be easily reviewed.

Also regenerate the data according to the, going forward, canonical
ordering algorithm.

* doc: fix hotdoc misuse for dynamically generated content

hotdoc has a native include feature for including files inline. Use this
to generate one file for each dynamically generated code block, and
include that file in Commands.md; see:
https://hotdoc.github.io/syntax-extensions.html#smart-file-inclusion-syntax

This permits us to move back to using the in-tree version of the hotdoc
*.md sources, thus fixing the incorrect inclusion of "builddir/" in the
"Edit on github" links which resulted from using copies as the source.

Fixes #8061

* doc: call the dummy file a "stamp" as it is a better known term

We added the _pathlib module to work around defeciencies in python 3.5's
implementation, since we now rely on 3.6 lets drop this

Subprojects that use the CMake PCH feature will cause
compilation/linker errors. The CMake PCH support
should thus be disabled until this can be properly
translated to meson.