1.4.0rc1

@ianna, @agoose77, and @jpivarski added to or corrected the documentation (PRs #891, #897, #925, #931, #943, #947, #951).

@agoose77 added ak.ptp (PR #890), added support for Arrow FixedSizeListType (PR #907), implemented ak.packed (PRs #918, #912, #937), fixed up ak.from_parquet (PRs #923, #935) and ak.unflatten (PR #922), corrected nbytes (PR #928), updated the deprecation system (PR #929), fixed ak.with_cache (PR #942), and did internal refactoring (PR #952).

@ianna added support for date-times (PRs #835, #915, #916), renamed TypedArrayBuilder → LayoutBuilder, thus completing it as it doesn't need to have a high-level interface (PR #924).

@ioanaif started the C++ → Python refactoring project with help from @agoose77 and @jpivarski (PRs #884, #895, #896, #914, #957, #958, #962).

@henryiii fixed an inexplicable bug in manylinux1 (PR #955), made the tests directory-independent (PR #953), and updated CMake infrastructure (PR #954). I think this will be the first deployment using GitHub Actions, rather than Azure! (No, that's PR #961, which is not included in this release.)

@jpivarski fixed random bugs (PRs #901, #904, #950) and made __getitem__ return Python str when they're strings (PR #874), pinned jaxlib<0.1.68 because of an intermittent MacOS segfault (PR #963; see connected discussion).