Native: fix contrast ratio to meet WCAG AA guidelines

[hugovk]

Before

The red #define _Py_SHARED_SHIFT 2 shown here, with class="cp" is #cd2828:

Has a contrast ratio of 3.04 which doesn't meet the WCAG AA minimum of 4.5:

And is flagged by Lighthouse:

After

Using Google's suggestion of #ff3a3a increases it to 4.59:

(For comparison, the AAA suggestion of #ff8888 is a bit pale, so let's go for AA.)

AAA

[birkenfeld]

Works for me. Thanks!