* 2.0 milestone: drop supported versions

The new policy is:

- Python >=3.6
- Sphinx >= 5
- HTML4 and HTML5 writer
- docutils >= 0.14, < 0.19

* Update roadmap after discussion

By the beginning of 2024 we should support newer Sphinx versions and HTML5 only.

* Remove Python 2.7 from CircleCI

* Run only supported versions on Python 3.11

* Update with feedback from review

* Update documentation to match our support

* Add Sphinx 6.2 that was missing

* Update tox/circleci to work

* Revert tox command

* Drop support for HTML4 and keep `-qa` testing

- drop support for HTML4 on 2.0
- keep `-qa` TOX environment to be able to compare feature branches
- emit `logger.errors` on Python 2 and html_writer4=True
- require Sphinx >= 5.0
- remove logic for old Sphinx versions

* Update Tox and CircleCI

* Update contributing page to match our plans

---------

Co-authored-by: Anthony Johnson <aj@ohess.org>

* Release 2.0.0rc1

* Update npm lock file

* Remove instructions "encouraging" to install directly from git

* Update changelog: Add reminder that people should not install directly from git

* Update docs/changelog.rst

---------

Co-authored-by: Manuel Kaufmann <humitos@gmail.com>

* Visual test with Firefox and tox

Allows developer to run

```
tox -e py310-sphinx72-firefox
```

This will run the tests and open a Firefox after building the demo site
showing the `demo/demo.html` page on it.

Then, you can also run:

```
tox -e py310-sphinx61-firefox
```

and compare the visual differences.

Idea copied from #1388
Closes #1388

* Default value for `DISPLAY` and `--new-tab`

* Simplify the idea :)

Use the latest config file style.

This attribute is now required by searchtools.js:
https://github.com/sphinx-doc/sphinx/blob/v7.2.2/sphinx/themes/basic/static/searchtools.js#L65

When it's not set, loading search results via AJAX will be broken
because the JS code will attempt to load paths like /undefinedfoo.html.

Co-authored-by: Manuel Kaufmann <humitos@gmail.com>

* Use `css_tag` helper to inject CSS files

* Typo

Closes #1516

Next development version is 2.0rc3 for now, but more likely 2.1rc1

* Fix #1522: fix attribute error if css is str

* Revert "Fix #1522: fix attribute error if css is str"

This reverts commit b22a77f.

* Use `css_tag` only for standard CSS files from Sphinx

... and keep using the custom `link` HTML tag for CSS files included via the
HTML theme option `extra_css_files`.

* Normalize template variables

---------

Co-authored-by: Manuel Kaufmann <humitos@gmail.com>
Co-authored-by: Anthony Johnson <aj@ohess.org>

Fix AttributeError when one of css_files is a string