Streamlink currently sets the version range of its `requests` dependency
to `>=2.26.0,<3.0`. `requests` however still defines its `urllib3`
dependency version range as `>=1.21.1,<1.27` in its `2.26.0`-`2.28.1`
releases, including its current state in git.

Recent fixes in Streamlink's HLS implementation unfortunately were made
with methods which are only supported by `urllib3 >=1.26.0`.
This obviously breaks installs with older versions of `urllib3`
installed which are still supported by the defined version range.

In order to avoid having to backport more `urllib3` stuff, moving it
from a transitive dependency and defining it as a direct dependency
with a specific version range set to `>=1.26.0` makes more sense when
fixing the issue. This doesn't conflict with the version range defined
by `requests` and forces a bump of old installs of `urllib3`.

`urllib3==1.26.0` was released on 2020-11-10, whereas
`requests==2.26.0`, was released on 2021-07-13, the lowest version
of the version range defined by Streamlink.

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In addition to defining the direct `urllib3` dependency, also remove
old compatibility workarounds/bugfixes from `plugin.api.http_session`.