Drop Python2.7 support

[ssteinbach]

As noted by #1094, VFX platform 2019 is the last VFX platform that includes python 2.7. This issue is to prepare for when we drop python 2.7 support.

• [ ] update check in setup.py:
  • [ ] https://github.com/AcademySoftwareFoundation/OpenTimelineIO/blob/78941ddf12a16e6b3495290e7c920ce674b1f342/setup.py#L186
  • [ ] https://github.com/AcademySoftwareFoundation/OpenTimelineIO/blob/78941ddf12a16e6b3495290e7c920ce674b1f342/setup.py#L295
  • [ ] https://github.com/AcademySoftwareFoundation/OpenTimelineIO/blob/78941ddf12a16e6b3495290e7c920ce674b1f342/setup.py#L336
• [ ] update README.md
• [ ] update files that reference 2.7, especially places marked # python2 or # python 2.7 (this is a rough list from grep)

  tests/test_builtin_adapters.py
  tests/test_console.py
  tests/test_timeline.py
  tests/test_cmx_3600_adapter.py
  tests/test_otioz.py
  tests/test_adapter_plugin.py
  tests/baselines/plugin_module/otio_mockplugin/__init__.py
  tests/test_hooks_plugins.py
  tests/test_examples.py
  contrib/opentimelineio_contrib/adapters/tests/test_hls_playlist_adapter.py
  src/py-opentimelineio/opentimelineio/adapters/file_bundle_utils.py
  src/py-opentimelineio/opentimelineio/adapters/otiod.py
  src/py-opentimelineio/opentimelineio/adapters/fcp_xml.py
  src/py-opentimelineio/opentimelineio/url_utils.py

• [ ] update .github/workflows/python-package.yml - python-package.yml
• [ ] update pybind11 to 2.10+
• [ ] try pyupgrade to move to newer syntax where available

[JeanChristopheMorinPerso]

Do we also want to update the code base to use newer syntax? If so then we can use https://github.com/asottile/pyupgrade to automatically upgrade code to use newer syntax. It also handles changing imports, and a bunch of different things. When running the tool, you basically tell it to target a specific Python version and it will take care of the rest.

[JeanChristopheMorinPerso]

We might also want to update Pybind11 to 2.10+.

[meshula]

it would be interesting to try pyupgrade and see what happens, and yes we'll want to update pybind11 to latest. we've been frozen for a whle

[ssteinbach]

@JeanChristopheMorinPerso great suggestion. It sounds like we want to do this after making release 0.15.0.

[ssteinbach]

(I added your notes as todo list items)