Do not use attrs 21.1.0

Attrs 21.1.0 does not work with Python 3.4. The release has been yanked
from pypi, but will still get used sometimes, so specify not to install
21.1.0.

Signed-off-by: Chris Marchbanks <[email protected]>

Author: csmarchbanks

tox.ini

@@ -6,6 +6,8 @@ envlist = coverage-clean,py2.7,py3.4,py3.5,py3.6,py3.7,py3.8,py3.9,pypy2.7,pypy3
 deps =
     coverage
     pytest
+; Python 3.4 fails if using 21.1.0.
+    attrs!=21.1.0
 
 [testenv:py2.7]
 deps =