Update TarArchiveOperations and equip with tests

[mih]

• More sensible __repr__ for ArchiveOperations
• Document methods
• More flexible handling of archive item names. open() and all other
  such methods can now accept item.name from __iter__ results
  directly. Based on the assumption that all TAR archives use member
  names in POSIX notation.
• Add unit tests with comprehensive coverage. Modelled after the
  ZipArchiveOperations tests in
  Add the class ZipArchiveOperations, which implements archive operations on zip-files #407 but with more
  context manager use.

Stuff from the development history of this PR

Interesting test failure pattern:

FAILED ../archive_operations/tests/test_tarfile.py::test_tararchive_iterator - AssertionError: assert 'test-archive/' in <datalad_next.archive_operations.tarfile.TarArchiveOperations object at 0x108f29fd0>

It is only happening with PY3.8, and not with PY3.9 or later. Likely a change in the tarfile module. Needs an investigation. The documentation is silent about such a change.

This also points out the need for a custom __repr__ implementation.

Update: I think the trailing-slash business is not needed:

appveyor@appveyor-vm:~$ /home/appveyor/venv3.7/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getnames())"
['test-archive', 'test-archive/123.txt', 'test-archive/123_hard.txt', 'test-archive/subdir', 'test-archive/subdir/onetwothree_again.txt', 'test-archive/onetwothree.txt']
appveyor@appveyor-vm:~$ /home/appveyor/venv3.8/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getnames())"
['test-archive', 'test-archive/123.txt', 'test-archive/123_hard.txt', 'test-archive/subdir', 'test-archive/subdir/onetwothree_again.txt', 'test-archive/onetwothree.txt']
appveyor@appveyor-vm:~$ /home/appveyor/venv3.9/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getnames())"
['test-archive', 'test-archive/123.txt', 'test-archive/123_hard.txt', 'test-archive/subdir', 'test-archive/subdir/onetwothree_again.txt', 'test-archive/onetwothree.txt']
appveyor@appveyor-vm:~$ /home/appveyor/venv3.10/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getnames())"
['test-archive', 'test-archive/123.txt', 'test-archive/123_hard.txt', 'test-archive/subdir', 'test-archive/subdir/onetwothree_again.txt', 'test-archive/onetwothree.txt']
appveyor@appveyor-vm:~$ /home/appveyor/venv3.11/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getnames())"
['test-archive', 'test-archive/123.txt', 'test-archive/123_hard.txt', 'test-archive/subdir', 'test-archive/subdir/onetwothree_again.txt', 'test-archive/onetwothree.txt']

appveyor@appveyor-vm:~$ /home/appveyor/venv3.7/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive'))"
<TarInfo 'test-archive' at 0x7f82cbfa0bb0>
appveyor@appveyor-vm:~$ /home/appveyor/venv3.8/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive'))"
<TarInfo 'test-archive' at 0x7f380a2bf7c0>
appveyor@appveyor-vm:~$ /home/appveyor/venv3.9/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive'))"
<TarInfo 'test-archive' at 0x7fe0ec693340>
appveyor@appveyor-vm:~$ /home/appveyor/venv3.10/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive'))"
<TarInfo 'test-archive' at 0x7f221e870100>
appveyor@appveyor-vm:~$ /home/appveyor/venv3.11/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive'))"
<TarInfo 'test-archive' at 0x7fde6ee504c0>

appveyor@appveyor-vm:~$ /home/appveyor/venv3.7/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive/'))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/appveyor/.localpython3.7.16/lib/python3.7/tarfile.py", line 1754, in getmember
    raise KeyError("filename %r not found" % name)
KeyError: "filename 'test-archive/' not found"
appveyor@appveyor-vm:~$ /home/appveyor/venv3.8/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive/'))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/appveyor/.localpython3.8.16/lib/python3.8/tarfile.py", line 1782, in getmember
    raise KeyError("filename %r not found" % name)
KeyError: "filename 'test-archive/' not found"
appveyor@appveyor-vm:~$ /home/appveyor/venv3.9/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive/'))"
<TarInfo 'test-archive' at 0x7fa7c6813340>
appveyor@appveyor-vm:~$ /home/appveyor/venv3.10/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive/'))"
<TarInfo 'test-archive' at 0x7fb3e8568100>
appveyor@appveyor-vm:~$ /home/appveyor/venv3.11/bin/python -c "import tarfile as tf; print(tf.open('test_archive.tar.xz').getmember('test-archive/'))"
<TarInfo 'test-archive' at 0x7fc59346c4c0>