Fix deepcopy of TaggedList

[eslavich]

The current behavior:

In [1]: from asdf.tagged import TaggedList; from copy import deepcopy

In [2]: foo = TaggedList([1, 2, 3])

In [3]: deepcopy(foo)
Out[3]: [1, 2, 3, 1, 2, 3]

In [4]: deepcopy(deepcopy(foo))
Out[4]: [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]

This has something to do with the TaggedList class inheriting from both list and UserList, but we can't drop list, because code elsewhere is probably relying upon this:

In [5]: isinstance(foo, list)
Out[5]: True

Which isn't the case when we inherit from UserList only -- users are expected to check collections.abc.Sequence instead. I'm in favor of dropping list eventually, but that seems like a 3.0 change.

This PR overrides __deepcopy__ to correct the issue with UserList. It does the same on UserDict, even though there wasn't a problem there, since the custom __deepcopy__ ended up being ~ 60% faster than whatever was happening without it. Overriding __copy__ offers no improvement, so I just left that alone.

Thanks to @jdavies-st for suggesting both __deepcopy__ and profiling UserDict!

[eslavich]

Turns out we also need a __copy__ override -- in Python < 3.8, copying the objects will result in MemoryError.

[codecov]

Codecov Report

Merging #788 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #788      +/-   ##
==========================================
+ Coverage   93.79%   93.87%   +0.07%     
==========================================
  Files          43       43              
  Lines        5028     5041      +13     
==========================================
+ Hits         4716     4732      +16     
+ Misses        312      309       -3     

Impacted Files	Coverage Δ
asdf/tagged.py	93.54% <100.00%> (+7.83%)	⬆️

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[jdavies-st]

Looks good! 👍

I wouldn't have guessed that changed between python 3.7 and 3.8. Is it documented somewhere?

[perrygreenfield]

Must be fun exploring these weird corners of behavior.