Support specifying the dandi redirector via an env var

[jwodder]

For dandi/redirector#42.

[codecov]

Codecov Report

Merging #581 (04b428c) into master (914a9cf) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #581      +/-   ##
==========================================
+ Coverage   83.08%   83.20%   +0.11%     
==========================================
  Files          59       59              
  Lines        5681     5685       +4     
==========================================
+ Hits         4720     4730      +10     
+ Misses        961      955       -6     

Flag	Coverage Δ
unittests	83.20% <100.00%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dandi/tests/fixtures.py	97.12% <ø> (ø)
dandi/consts.py	100.00% <100.00%> (ø)
dandi/tests/test_dandiarchive.py	100.00% <100.00%> (ø)
dandi/tests/test_utils.py	100.00% <100.00%> (ø)
dandi/download.py	87.65% <0.00%> (+1.85%)	⬆️

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[jwodder]

Note: When I run the tests with DANDI_REDIRECTOR_BASE=http://localhost:8079, everything succeeds except for test_utils.py::test_server_info, which keeps getting a "connection refused" error.

[yarikoptic]

I guess we should still have this one finished up and whatever connection issue is figured out and proceed, right?

[jwodder]

@yarikoptic Merge conflicts resolved; this PR should be ready now. The connection issue was resolved by dandi/redirector#45. After this PR is published in a release, dandi/redirector#47 can be merged.

[yarikoptic]

well, to not hold off other PRs, let's merge and release then!