Allow compressed metadata in flight

[jku]

from commit description:

This commit tries to deal with two interests:
* metadata is highly repetitive and compressible: allowing compression
would be good
* there may be broken web servers (see
https://github.com/pypa/pip/blob/404838abcca467648180b358598c597b74d568c9/src/pip/_internal/download.py#L842)
that have problems with compression on already compressed target files

We can make things better for that first interest while we have no real
data for the second interest -- our current workarounds to avoid
compression are based on hearsay, not testing.

Now that individual fetchers are possible I suggest we simplify
ngclient and allow compression. As an example the pip Fetcher
could still use the pip response chunking code with all their
workarounds -- pip certainly has better capability to maintain
a mountain of workarounds and also has endless amounts of real-world
testing compared to python-tuf.

Details:
* Stop modifying Accept-Encoding (Requests default includes gzip)
* Don't use response.raw in RequestsFetcher as there is no need anymore
* Fix issue in test_session_get_timeout(): it's not mocking the error that requests really raises in this case

Fixes #1251

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[coveralls]

Pull Request Test Coverage Report for Build 1745548324

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.0008%) to 97.691%

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Totals
Change from base Build 1745219259:	-0.0008%
Covered Lines:	4086
Relevant Lines:	4167

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💛 - Coveralls

[kairoaraujo]

LGTM
I'm not sure about the third commit as well.
As we talked about before, requests support it, so we are testing something outside the TUF domain.

[jku]

Marking non-draft with only these changes:

• rebased on develop
• removed the last commit (tests: Add gzip support to simple_server): we don't want to test every Requests feature that is essentially opaque to us. The commit is still conserved in another branch: https://github.com/jku/python-tuf/tree/gzip-simple-server

[lukpueh]

👍 Thanks for cleaning up!

As the comment above says, I introduced this workaround for testing on Windows in Python2 (7dbb30a). So this has become irrelevant quite a while ago.

[jku]

modified commit message with an explanation of why we no longer use requests.Response.raw (which is really a urllib3.response.HTTPResponse)