Drop Python 3.6 support

[twm]

No description provided.

[twm]

Well that's a new one:

pypy3-twisted_lowest: commands[1]> python -Wall /home/runner/work/treq/treq/.tox/pypy3-twisted_lowest/bin/coverage run -p /home/runner/work/treq/treq/.tox/pypy3-twisted_lowest/bin/trial treq
Error in cpyext, CPython compatibility layer:
PyThreadState_Swap() cannot be used to switch to another
RPython traceback:
  File "pypy_module_cpyext_2.c", line 26465, in unwrapper__StdObjSpaceConst_PyThreadStatePtr_sta
  File "pypy_module_cpyext_1.c", line 65525, in PyThreadState_Swap
Fatal RPython error: AssertionError
different PyThreadState right now
pypy3-twisted_lowest: exit -6 (5.73 seconds) /home/runner/work/treq/treq> python -Wall /home/runner/work/treq/treq/.tox/pypy3-twisted_lowest/bin/coverage run -p /home/runner/work/treq/treq/.tox/pypy3-twisted_lowest/bin/trial treq pid=3614
.pkg: _exit> python /opt/hostedtoolcache/PyPy/3.7.13/x64/site-packages/pyproject_api/_backend.py True setuptools.build_meta
  pypy3-twisted_lowest: FAIL code -6 (161.85=setup[155.05]+cmd[1.07,5.73] seconds)
  evaluation failed :( (162.10 seconds)

PyPy 3.7 appears to be EOL, so I'll drop it.