…wnload (#488)

Populate properties during download functions via Blob._extract_headers_from_download.

Additionally, update documentation for running system tests to use Python 3.8 instead of Python 3.7 to align with nox.

Closes #490.

Support conditional requests based on ETag for read operations (`reload`, `exists`, `download_*`). My own testing seems to indicate that the JSON API does not support ETag If-Match/If-None-Match headers on modify requests (`patch`, `delete`, etc.), please correct me if I am mistaken.

This part two of #451. Part one in #488.
Fixes #451 🦕

Closes #493

Also, actually test those minimum versions using constraints file.

Closes #494

* fix: handle cases where data is a dict in is_etag_in_json(data)

* address comments and accept proposed changes

* revise is_etag_in_json to check dict data

* revise docstring wording

* revise docstrings

* rename conditional predicate to is_etag_in_data

Closes #469.

Test hygeine: avoid importing module-under-test at module scope

Closes #498.

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>