tox.ini: make "tox -e pypy" etc work

This uses internal env vars for deps, allowing to install them always
(unpinned).

tox.ini

@@ -10,21 +10,27 @@ envlist =
 extras = testing
 setenv =
     PYTHONUNBUFFERED=yes
+
+    # Use env vars for (optional) pinning of deps.
+    t310: _PYTEST_VERSION="==3.10.1"
+    t40:  _PYTEST_VERSION="==4.0.2"
+    t41:  _PYTEST_VERSION="==4.1.1"
+    t43:  _PYTEST_VERSION="==4.3.1"
+    t44:  _PYTEST_VERSION="==4.4.2"
+    t45:  _PYTEST_VERSION="==4.5.0"
+
+    {t310,t40,t41,t43}: _PYTESTXDIST_VERSION="==1.27.0"
+    {t44,t45}: _PYTESTXDIST_VERSION="==1.28.0"
+
+    c44: _COVERAGE_VERSION="==4.4.2"
+    c45: _COVERAGE_VERSION="==4.5.3"
+
 passenv =
     *
 deps =
-    t310: pytest==3.10.1
-    t40: pytest==4.0.2
-    t41: pytest==4.1.1
-    t43: pytest==4.3.1
-    t44: pytest==4.4.2
-    t45: pytest==4.5.0
-
-    {t310,t40,t41,t43}: pytest-xdist==1.27.0
-    {t44,t45}: pytest-xdist==1.28.0
-
-    c44: coverage==4.4.2
-    c45: coverage==4.5.3
+    pytest{env:_PYTEST_VERSION:}
+    pytest-xdist{env:_PYTESTXDIST_VERSION:}
+    coverage{env:_COVERAGE_VERSION:}
 pip_pre = true
 commands =
     pytest {posargs:-vv}