[Snyk] Security upgrade python from 3-slim-stretch to 3.7-slim-stretch

[snyk-bot]

Keeping your Docker base image up-to-date means you’ll benefit from security fixes in the latest version of your chosen image.

Changes included in this PR

• _action_files/fastpages-nbdev.Dockerfile

We recommend upgrading to python:3.7-slim-stretch, as this image has only 87 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected.

Some of the most important vulnerabilities in your base image include:

Severity	Priority Score / 1000	Issue	Exploit Maturity
	571	Out-of-bounds Write SNYK-DEBIAN9-GLIBC-356851	Mature
	571	Out-of-bounds Write SNYK-DEBIAN9-GLIBC-356851	Mature
	571	Out-of-bounds Write SNYK-DEBIAN9-GLIBC-356851	Mature
	614	Integer Overflow or Wraparound SNYK-DEBIAN9-OPENSSL-1075328	No Known Exploit
	614	Out-of-bounds Read SNYK-DEBIAN9-OPENSSL-1569399	No Known Exploit

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Latest commit: 23fd3da

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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