docs(changelog): record the post-#719 defect wave

- 22 new 1.5.0 entries for the 30 PRs merged since b10e795
  (#1106-#1158), with related fixes grouped per the #918 ruling;
  three marked breaking to match their labels (#1116, #1138, #1145)
- regenerate CHANGELOG.md with util/changelog_md.py
- process.rst: entry-count pin re-measured, 167 -> 189
- pep.rst: note the plist round-trip test skipped on the upstream
  dictdumper fractional-second <date> defect (DictDumper#122)

tests/project: conventions_doc_claims, changelog_md and
documentation_claims pass; changelog_md.py --check is in step.

Author: JarryShaw