Reduce setuptools dep to >=11.3

We don't necessarily need this ourselves (__init__.py's version-checking
code is the only thing in tahoe per se that uses setuptools, and our
setup.py's use of setuptools isn't something that install_requires= can
say anything about). But at least one old environment failed because a
sub-dependency needed a newer version than Tahoe asked for. I'm not sure
if this ought to be here, but it may help for a transitional period
until these ancient environments get updated.

closes ticket:2744

src/allmydata/_auto_deps.py

@@ -16,7 +16,11 @@
 # >=, <= and != operators.)
 
 install_requires = [
-    "setuptools >= 20.3", # current version at this moment
+    # we don't need much out of setuptools, but the __init__.py stuff does
+    # need pkg_resources . We use >=11.3 here because that's what
+    # "cryptography" requires (which is a sub-dependency of TLS-using
+    # packages), so there's no point in requiring less.
+    "setuptools >= 11.3",
 
     "zfec >= 1.1.0",