remove basepython usages

tox.ini

@@ -5,15 +5,6 @@ envlist =
     docs,coverage
 
 [testenv]
-basepython =
-    py27: python2.7
-    py34: python3.4
-    py35: python3.5
-    py36: python3.6
-    py37: python3.7
-    pypy: pypy
-    pypy3: pypy3
-
 commands =
     py.test --cov --cov-report= {posargs:}
 
@@ -24,8 +15,7 @@ extras =
     testing
 
 [testenv:coverage]
-skip_install = True
-basepython = python3.6
+skip_install = true
 commands =
     coverage combine
     coverage report --fail-under=100
@@ -35,7 +25,6 @@ setenv =
     COVERAGE_FILE=.coverage
 
 [testenv:docs]
-basepython = python3.5
 whitelist_externals =
     make
 commands =
@@ -45,8 +34,7 @@ extras =
     docs
 
 [testenv:lint]
-skip_install = True
-basepython = python3.6
+skip_install = true
 commands =
     flake8 src/pyramid_retry/
     python setup.py check -r -s -m
@@ -58,7 +46,6 @@ deps =
 
 [testenv:build]
 skip_install = true
-basepython = python3.6
 commands =
     # clean up build/ and dist/ folders
     python -c 'import shutil; shutil.rmtree("dist", ignore_errors=True)'