Release notes and changelog for 6.1.0

[mdickinson]

Building on @kitchoi's work, here's a first draft of release notes and changelog.

There's a bit of duplication between the prose version of the changes and the PR-by-PR version, especially with respect to the deprecations. I'm not sure how best to combine them, or whether to leave as-is.

Closes #1101.

For future use, here's the quick-and-dirty Python script I'm using to check for missing changelog entries:

import re
import subprocess

cmd = ["git", "log", "6.0.0..origin/master", "--pretty=* %s"]
git_log = subprocess.check_output(cmd).decode("utf-8")

pr_numbers = {
    re.search(r"\((#\d+)\)$", log_entry).group(1): log_entry
    for log_entry in git_log.splitlines()
}

with open("CHANGES.rst", "r", encoding="utf-8") as f:
    changelog = f.read()

for pr_number in pr_numbers:
    if pr_number not in changelog:
        print("Missing PR: ", pr_numbers[pr_number])

[mdickinson]

This will be updated upon actual release; there's already an entry in the release checklist to remind us to do this.

[mdickinson]

This is mostly done, but:

• waiting on Add intersphinx extension #1136 and Add user manual for observe #1060
• needs updating for recently-merged PRs.

[kitchoi]

LGTM. Two minor suggestions.

[kitchoi]

Looking great!

[mdickinson]

This change was already present in 6.0.

[mdickinson]

#855 went into Traits 6.0; removing here

[mdickinson]

Already in 6.0.