Change Tox configuration: use a clearer dependency declaration for ea…

…ch version of Python.

tox.ini

@@ -33,13 +33,23 @@ passenv =
     *
 usedevelop = false
 deps =
-    py27,py34,py35: pip >= 9.0.3
-    py27,py34: PyTest < 5
-    py35,py36,py37,py38,py39,py310: PyTest
-    py27,py34: PyTest-Cov < 2.6
-    py35,py36,py37,py38,py39,py310: PyTest-Cov
+    py27: pip >= 9.0.3, < 21
+    py27: PyTest < 5
+    py27: PyTest-Cov < 2.6
+    py27: Mock
+
+    py34: pip >= 9.0.3
+    py34: PyTest < 5
+    py34: PyTest-Cov < 2.6
     py34: attrs < 21.1
-    Mock
+
+    py35: pip >= 9.0.3
+    py35: PyTest
+    py35: PyTest-Cov
+
+    py36,py37,py38,py39,py310: PyTest
+    py36,py37,py38,py39,py310: PyTest-Cov
+
     lxml3.8: lxml ~= 3.8.0
     lxml4.0: lxml ~= 4.0.0
     lxml4.1: lxml ~= 4.1.0
@@ -48,7 +58,9 @@ deps =
     lxml4.4: lxml ~= 4.4.0
     lxml4.5: lxml ~= 4.5.0
     lxml4.6: lxml ~= 4.6.0
+
     xmldiff
+
 commands =
     {posargs:py.test --cov --cov-report=term-missing --cov-append -vv tests}