[python] set user-agent

[ivirshup]

This PR sets a user agent on requests from the python api, addressing part of #997

• Adds a new user agent to all requests made by this library. Format is cellxgene-census-python/{VERSION}
• Defines a new environment variable CELLXGENE_CENSUS_USERAGENT whose value will be added to the user agent if defined. This allows us to set the flag on internal systems and figure out internal vs external usage of the resource.
• Tests for all this are in test_user_agent. This is the most complicated part.
  • For each test session we set up an ephemeral proxy server that intercepts and logs all requests
  • We turn off SSL verification for all requests so that we can do the interception
  • This is exposed through a fixture that gives access to the logged requests

Could do

• [ ] Some centralization of environment variables
• [ ] Attention payed to mypy.

Most of the typing problems are in the tests. I think there's very little value to adding more type info here, and it gets quite complicated since most of it is in fixtures. I don't think doing much more is worth it, so would like to ignore these.

Probably should do:

• [x] Set a custom user agent in the testing environment (though the version number should be .dev for all of these). Idk what this should be.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 92.09040% with 14 lines in your changes missing coverage. Please review.

Project coverage is 91.25%. Comparing base (8d1e103) to head (3f5a42d). Report is 3 commits behind head on main.

Files	Patch %	Lines
...nsus/src/cellxgene_census/_testing/logger_proxy.py	42.10%	11 Missing :warning:
...i/python/cellxgene_census/tests/test_user_agent.py	98.55%	2 Missing :warning:
...hon/cellxgene_census/src/cellxgene_census/_util.py	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1193 +/- ## ========================================== + Coverage 91.11% 91.25% +0.13% ========================================== Files 77 79 +2 Lines 5923 6141 +218 ========================================== + Hits 5397 5604 +207 - Misses 526 537 +11 ``` | [Flag](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/1193/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/1193/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | `91.25% <92.09%> (+0.13%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ivirshup]

For test coverage on the logger_proxy.py file, that code isn't executed by the test process so I wouldn't expect coverage.