Fix for publishing prereleases

[jarrodmillman]

Fix #973.

This should fix https://github.com/pydata/pydata-sphinx-theme/actions/runs/3182801089.

I tested on my laptop and it works. I am not sure if the leading slash is ever needed. The tests pass when it is removed, but I am not sure why it was there to begin with.

In [1]: from pathlib import Path

In [2]: srcdir = '/home/jarrod/src/pydata-sphinx-theme'

In [3]: json_url = '/_static/switcher.json'

In [4]: Path(srcdir, json_url)
Out[4]: PosixPath('/_static/switcher.json')

In [5]: json_url = '_static/switcher.json'

In [6]: Path(srcdir, json_url)
Out[6]: PosixPath('/home/jarrod/src/pydata-sphinx-theme/_static/switcher.json')

The error is triggered in src/pydata_sphinx_theme/__init__.py here:

        if urlparse(json_url).scheme in ["http", "https"]:
            content = requests.get(json_url).text
        else:
            content = Path(env.srcdir, json_url).read_text()

when Path(env.srcdir, json_url).read_text() sets content to '/_static/switcher.json instead of something like /home/jarrod/src/pydata-sphinx-theme/_static/switcher.json.

[jarrodmillman]

I will make 0.11rc2 if this fix looks good.

[choldgraf]

Lgtm!