pin requests version in py26-oldest (#3803)

tox.ini

@@ -31,6 +31,8 @@ setenv =
 
 # cffi<=1.7 is required for oldest tests due to
 # https://bitbucket.org/cffi/cffi/commits/18cdf37d6b2691301a15b0e54f49757ebd4ed0f2?at=default
+# requests<=2.11.1 required for py26-oldest tests due to
+# https://github.com/shazow/urllib3/pull/930
 deps =
     py{26,27}-oldest: cffi<=1.7
     py{26,27}-oldest: cryptography==0.8
@@ -38,6 +40,7 @@ deps =
     py{26,27}-oldest: dnspython>=1.12
     py{26,27}-oldest: psutil==2.1.0
     py{26,27}-oldest: PyOpenSSL==0.13
+    py26-oldest: requests<=2.11.1
 
 [testenv:py33]
 commands =