Update syntax for Sphinx versions

Author: humitos

tox.ini

@@ -1,19 +1,19 @@
 [tox]
 envlist =
   docs
-  py{36,37,38}-sphinx{185,201,210,220,231,244}
+  py{36,37,38}-sphinx{18,20,21,22,23,24}
 
 [testenv]
 deps =
   pytest
   pdbpp
   .
-  sphinx185: sphinx==1.8.5
-  sphinx201: sphinx==2.0.1
-  sphinx210: sphinx==2.1.0
-  sphinx220: sphinx==2.2.0
-  sphinx231: sphinx==2.3.1
-  sphinx244: sphinx==2.4.4
+  sphinx18: sphinx~=1.8.0
+  sphinx20: sphinx~=2.0.0
+  sphinx21: sphinx~=2.1.0
+  sphinx22: sphinx~=2.2.0
+  sphinx23: sphinx~=2.3.0
+  sphinx24: sphinx~=2.4.0
 commands = pytest {posargs}
 
 [testenv:py38-sphinx244]