Pin selenium to avoid doctest breakage from deprecation

[joelostblom]

Altair's doctests are currently failing due to a deprecation in Selenium 4.3.0 which is a dependency of altair-saver (as reported in altair-viz/altair_saver#104). Until there is a new release of altair-saver that addresses this (maybe with altair-viz/altair_saver#105), this PR make sure we can run doctests by pinning the version of selenium.

There will likely still be confusion among people who use the altair-saver library directly until it sees a new release since selenium 4.3.0 seems to be the default version installed with altair-saver currently.

[mattijn]

If I try locally I need double quotes.

pip install "selenium<4.3.0"

Also documented here: https://docs.python.org/3/installing/index.html#basic-usage:

When using comparator operators such as >, < or some other special character which get interpreted by shell, the package name and the version should be enclosed within double quotes

[joelostblom]

Good catch! This is unexpected for me since most Unix shells expand variables within double quotes but not single quotes, and I would expect us to want the string to be passed literally to pip without any possible shell expansion taking place. I wonder if it is an OS-specific issue but as you pointed out, let's follow what the Python docs say. I updated to double quotes and will merge if all the tests pass.