Add script to generate draft changelog entries

[JukkaL]

The script format changelog entries based on commit history and has some rules to filter out some changes, such as typeshed sync and changes cherry-picked to the previous release branch.

Example of how to run it:

$ python misc/generate_changelog.py 1.7
Generating changelog for 1.7
Previous release was     1.6
Merge base: d7b24514d7301f86031b7d1e2215cf8c2476bec0
NOTE: Drop "Fix crash on ParamSpec unification (for real)", since it was in previous release branch
NOTE: Drop "Fix crash on ParamSpec unification", since it was in previous release branch
NOTE: Drop "Fix mypyc regression with pretty", since it was in previous release branch
NOTE: Drop "Clear cache when adding --new-type-inference", since it was in previous release branch
NOTE: Drop "Match note error codes to import error codes", since it was in previous release branch
NOTE: Drop "Make PEP 695 constructs give a reasonable error message", since it was in previous release branch
NOTE: Drop "Fix ParamSpec inference for callback protocols", since it was in previous release branch
NOTE: Drop "Try upgrading tox", since it was in previous release branch
NOTE: Drop "Optimize Unpack for failures", since it was in previous release branch

 * Fix crash on unpack call special-casing (Ivan Levkivskyi, PR [16381](https://github.com/python/mypy/pull/16381))
 * Fix file reloading in dmypy with --export-types (Ivan Levkivskyi, PR [16359](https://github.com/python/mypy/pull/16359))
 * Fix daemon crash caused by deleted submodule (Jukka Lehtosalo, PR [16370](https://github.com/python/mypy/pull/16370))
...