CI fix for "Testing nbclassic" on macos / pypy

[ericsnekbytes]

This PR proposes bumping the version of pypy being tested to 3.8 (which allows our CI tests to pass), then possibly opening a new issue for revisiting the pypy testing strategy in the future. We should think about the purpose of this test:

• Users of pypy3.7 on macos will have a hard time using notebook at all, as cryptography (a dependency) does not provide a wheel for pypy3.7 on macos
• Wheels are already available for needed pypy3.8 dependencies so it's a simple one line change (our CI test passes with pypy3.8)
• If we bump to pypy3.8, we do not need to maintain a build toolchain (on the CI system/runners) for the cryptography library / dependencies, which involves configuring a rust build toolchain, package manager and openssl
• We don't want to debug a from-source build of cryptography with this test, we just want a reasonable expectation that notebook works with pypy, so this keeps the test focused on the right things

[ericsnekbytes]

@Zsailer @blink1073 @jtpio @echarles @RRosio Does anyone have any comments on the proposal above?

[blink1073]

Makes sense to me

[echarles]

Sounds good to me. @ericsnekbytes Could you please undraft this PR. Lets wait a few more days to gather any more feedback before merging.

[ericsnekbytes]

Sounds good to me. @ericsnekbytes Could you please undraft this PR. Lets wait a few more days to gather any more feedback before merging.

Just un-drafted the PR.

[echarles]

LGTM Thx @ericsnekbytes