BugFix: Upgrade from Python 3.7 to 3.8

[jawnsy]

Description

<! -- What is it meant to do? -->

We have a build error when running with 3.7, which may be due to a change on the Netlify side:

10:44:49 PM: Starting build script
10:44:49 PM: Installing dependencies
10:44:49 PM: /opt/build-bin/run-build-functions.sh: line 223: /opt/buildhome/python3.7/bin/activate: No such file or directory
10:44:49 PM: Error setting python version from runtime.txt
10:44:49 PM: Please see https://github.com/netlify/build-image/blob/focal/included_software.md for current versions

Since Python is likely only used during the build, upgrading is probably safe, though someone else should confirm. In Cloud 1, all of our static artifacts are served out of an Google Cloud Storage bucket and are static content only.

Linked Issues

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Tests and performance

• Changes to any .js files are covered by existing tests or this PR adds new tests (if not please explain why below)
• No new packages are added or package size and performance considerations are discussed below
• PR Title fits our changelog format

Releases/Changelog cuts only

• Completed the QA Checklist in staging

[zanieb]

See also: PrefectHQ/prefect#6536; this resolved the same error in v1 documentation builds.