Allow overriding the interpreter from environment.

Author: ionelmc

ci/templates/tox.ini

@@ -108,7 +108,7 @@ deps = coverage
 
 {% for env, config in tox_environments|dictsort %}
 [testenv:{{ env }}]
-basepython = {{ config.python }}
+basepython = {env:TOXPYTHON:{{ config.python }}}
 {% if config.cover or config.env_vars %}
 setenv =
     {[testenv]setenv}

tox.ini

@@ -114,7 +114,7 @@ usedevelop = true
 deps = coverage
 
 [testenv:2.6]
-basepython = python2.6
+basepython = {env:TOXPYTHON:python2.6}
 setenv =
     {[testenv]setenv}
     WITH_COVERAGE=yes
@@ -128,10 +128,10 @@ deps =
     pytest-cover
 
 [testenv:2.6-nocover]
-basepython = python2.6
+basepython = {env:TOXPYTHON:python2.6}
 
 [testenv:2.7]
-basepython = python2.7
+basepython = {env:TOXPYTHON:python2.7}
 setenv =
     {[testenv]setenv}
     WITH_COVERAGE=yes
@@ -146,13 +146,13 @@ deps =
     Django
 
 [testenv:2.7-nocover]
-basepython = python2.7
+basepython = {env:TOXPYTHON:python2.7}
 deps =
     {[testenv]deps}
     Django
 
 [testenv:3.3]
-basepython = python3.3
+basepython = {env:TOXPYTHON:python3.3}
 setenv =
     {[testenv]setenv}
     WITH_COVERAGE=yes
@@ -167,13 +167,13 @@ deps =
     Django
 
 [testenv:3.3-nocover]
-basepython = python3.3
+basepython = {env:TOXPYTHON:python3.3}
 deps =
     {[testenv]deps}
     Django
 
 [testenv:3.4]
-basepython = python3.4
+basepython = {env:TOXPYTHON:python3.4}
 setenv =
     {[testenv]setenv}
     WITH_COVERAGE=yes
@@ -188,13 +188,13 @@ deps =
     Django
 
 [testenv:3.4-nocover]
-basepython = python3.4
+basepython = {env:TOXPYTHON:python3.4}
 deps =
     {[testenv]deps}
     Django
 
 [testenv:pypy]
-basepython = pypy
+basepython = {env:TOXPYTHON:pypy}
 setenv =
     {[testenv]setenv}
     WITH_COVERAGE=yes
@@ -209,7 +209,7 @@ deps =
     Django
 
 [testenv:pypy-nocover]
-basepython = pypy
+basepython = {env:TOXPYTHON:pypy}
 deps =
     {[testenv]deps}
     Django