TO Postinstall: Check if hashlib.scrypt() exists before calling it

[zrhoffman]

Fixes #6285 by, rather than assuming that Python 3 includes hashlib.scrypt(), checking whether it exists, and if not, using our pure Python implementation of Scrypt.

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Which Traffic Control components are affected by this PR?

• Traffic Ops - Postinstall

What is the best way to verify this PR?

Verify that _postinstall.hash_pass() works in Python 3 on CentOS 7.

<<'DOCKER_COMMANDS' docker run --rm -iv$(pwd):/opt -w/opt centos:7 bash
yum -y install python3 &&
<<'PYTHON_COMMANDS' PYTHONPATH=traffic_ops/install/bin python3
import _postinstall
print(_postinstall.hash_pass('mysecretpassword'))
PYTHON_COMMANDS
DOCKER_COMMANDS

Expected output:

SCRYPT:16384:8:1:/clKimOLhQKGKtLV1rH1UNyykUQ0VRVdRZj+/I8YQ940vqkHYh5OiAaGMVDUfl8PXfkHnyoq3XRrWxmLOtLCLw==:IL/i6VQqT5rWyOf2eKE1/7GY5fia1AyOLSJNL8HkVhArzU7R0S2joJ4ls+bIQdkCx23PknEmMIm4+41q99GmZw==

If this is a bugfix, which Traffic Control versions contained the bug?

• 6.0.0
• master

PR submission checklist

• Existing tests are sufficient
• Bugfix, no documentation necessary
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[ocket8888]

This looks good to me, CiaB and postinstall tests still passing, just needs a conflict fix

[zrhoffman]

Rebased to fix the merge conflict in CHANGELOG.md