ci: test oldest supported attrs [ci skip]

Currently fails due to hypothesis(?):

    % tox -e py37-oldattrs
    .package create: /tmp/tox…/Vcs/pytest/.package
    .package installdeps: setuptools>=40.0, setuptools-scm, wheel
    py37-oldattrs create: /tmp/tox…/Vcs/pytest/py37-oldattrs
    py37-oldattrs installdeps: attrs==17.4.0
    py37-oldattrs inst: /tmp/tox…/Vcs/pytest/.tmp/package/1/pytest-5.2.1.dev12+g8c9ea5e05.d20191006.tar.gz
    py37-oldattrs installed: argcomplete==1.10.0,atomicwrites==1.3.0,attrs==17.4.0,certifi==2019.9.11,chardet==3.0.4,elementpath==1.2.1,hypothesis==4.38.3,idna==2.8,importlib-metadata==0.23,mock==3.0.5,more-itertools==7.2.0,nose==1.3.7,packaging==19.2,pluggy==0.13.0,py==1.8.0,pyparsing==2.4.2,pytest==5.2.1.dev12+g8c9ea5e05.d20191006,requests==2.22.0,six==1.12.0,urllib3==1.25.6,wcwidth==0.1.7,xmlschema==1.0.14,zipp==0.6.0
    py37-oldattrs run-test-pre: PYTHONHASHSEED='2261891356'
    py37-oldattrs run-test: commands[0] | pytest
    Traceback (most recent call last):
      …
      File "/tmp/tox…/Vcs/pytest/py37-oldattrs/lib/python3.7/site-packages/_pytest/assertion/rewrite.py", line 142, in exec_module
        exec(co, module.__dict__)
      File "/tmp/tox…/Vcs/pytest/py37-oldattrs/lib/python3.7/site-packages/hypothesis/internal/conjecture/shrinker.py", line 1445, in <module>
        @attr.s(slots=True, eq=False)
    TypeError: attrs() got an unexpected keyword argument 'eq'

Author: blueyed

setup.py

@@ -5,7 +5,7 @@
 INSTALL_REQUIRES = [
     "py>=1.5.0",
     "packaging",
-    "attrs>=17.4.0",
+    "attrs>=17.4.0",  # should match oldattrs tox env.
     "more-itertools>=4.0.0",
     "atomicwrites>=1.0",
     'pathlib2>=2.2.0;python_version<"3.6"',

tox.ini

@@ -41,6 +41,7 @@ setenv =
     xdist: _PYTEST_TOX_POSARGS_XDIST=-n auto
 extras = testing
 deps =
+    oldattrs: attrs==17.4.0
     numpy: numpy
     pexpect: pexpect
     pluggymaster: git+https://github.com/pytest-dev/pluggy.git@master