Awkward

Another batch of [November bug-fixes](https://github.com/scikit-hep/awkward-1.0/projects/2):

nsmith- made mixin classes derived from a decorator pickleable (542).

ianna fixed a bug in sort/argsort (524) and concatenate (548).

reikdas added a script to clean up generated files (556).

jpivarski generalized `ak.where` to take a non-NumPy `condition` (544), implemented `__contains__` in and out of Numba (547), `np.array(ak.Array)` in Numba (550), removed the right-broadcast from `ak.with_field` (553), implemented `ak.to_regular` and `ak.from_regular` (551), updated the GitHub issue templates, and fixed a bug affecting Coffea (https://github.com/CoffeaTeam/coffea/issues/372, PR #558).

Mostly bug-fixes, but a few new functions:

* PR 495: reikdas added a test to ensure kernel signatures match specification, are in order
* PR 517: masked `__getitem__` on empty arrays
* PR 518: fixed `ak.to_pandas` with IndexedArrays and other nodes
* PR 519: `ak.local_index`
* PR 520: actually removed long-deprecated Pandas code
* PR 525: fixed the definition of `Form::getitem_field`
* PR 526: infinite recursion bug in Arrow translation
* PR 527: definition of "mergeable" with parameters (different `__doc__` is mergeable, different `__record__` is not)
* PR 535: `ak.zeros_like`, `ak.ones_like`, `ak.full_like`
* PR 536: enforce minimum pyarrow version (2.0.0)
* PR 537: fixed matrix multiplication (multiply many differently sized matrices with one ``!)
* PR 539: ianna implemented `ak.concatenate` for `axis > 0`!
* PR 540: performance issues in `__setitem__`

Calling ufuncs on any custom types (not just records) will raise errors unless a customization is defined for exactly that ufunc and type. Some custom types, like "categorical," need to handle arbitrary ufuncs, so I also added a "apply_ufunc" interface to define behaviors across the board. (513)

Added copy and deepcopy, both with the Python interface (`copy` module) and with the NumPy interface (`np.copy`). (514)

Fixed the cache-handling of version 0.4.2. (515)

Mutable `ak.Array.cache` has been replaced with immutable `ak.Array.caches`, which always contains all the VirtualArray caches in the whole layout; it no longer needs to be maintained by users.

This is an interface-breaking change (with deprecation warnings), and a new release is needed to bring uproot4 up to date with the change.

Broadcasting ufuncs across records is now deprecated, and will be removed in version 1.0.0 (December 1, 2020).

Fixed two bugs (501 and 499).

Supports Python 3.9. (Now MacOS only supports Pythons 2.7, 3.7-3.9 (3.6 dropped). All other operating systems support Pythons 2.7, 3.5-3.9)

Based on [pybind 2.6.0](https://github.com/pybind/pybind11/releases/tag/v2.6.0). This is the first jump in pybind11 version in a while.