This temporarily fixes the issue of randomly failing tests on
Windows which seems to be related to the latest requests version
2.26 and requests-mock, where mocked request responses take an
random/arbitrary time to return data, which makes the
EventedHLSStreamWriter tests miss their time frames when writing
HLS segment data to the output buffer.
Since Windows builds of Streamlink don't rely on system python
packages like on most Linux distros for example, downgrading and
forcing an older version of requests on Windows should be fine.
This commit thus partly reverts  35cb8d9  and  29a25fc  which fixed
the broken dependencies of the Windows installer and skipped the
randomly failing tests on Windows.