Move testtools requirement to a "streams" extra

Now that the core of `fixtures` no longer uses `testtools`, all that
remains to make it optional and break the cyclic requirement is to deal
with its use by streams fixtures.  These intrinsically rely on
`testtools`, but are also very much optional, so move the requirement to
a new `fixtures[streams]` extra.

Fixes #38.

3 files changed, 13 insertions, 3 deletions

diff --git a/README.rst b/README.rst
index b60d993..9faafa3 100644
--- a/README.rst
+++ b/README.rst
@@ -336,6 +336,8 @@ in test failure descriptions. Very useful in combination with MonkeyPatch.
   ...     pass
   >>> fixture.cleanUp()
 
+This requires the ``fixtures[streams]`` extra.
+
 EnvironmentVariable
 +++++++++++++++++++
 
@@ -450,6 +452,8 @@ in test failure descriptions. Very useful in combination with MonkeyPatch.
   ...     pass
   >>> fixture.cleanUp()
 
+This requires the ``fixtures[streams]`` extra.
+
 TempDir
 +++++++
 
diff --git a/fixtures/_fixtures/streams.py b/fixtures/_fixtures/streams.py
index 7686376..c0fb60a 100644
--- a/fixtures/_fixtures/streams.py
+++ b/fixtures/_fixtures/streams.py
@@ -22,7 +22,6 @@ __all__ = [
 import io
 
 from fixtures import Fixture
-import testtools
 
 
 class Stream(Fixture):
@@ -42,10 +41,14 @@ class Stream(Fixture):
         self._stream_factory = stream_factory
 
     def _setUp(self):
+        # Available with the fixtures[streams] extra.
+        from testtools.content import content_from_stream
+
         write_stream, read_stream = self._stream_factory()
         self.stream = write_stream
-        self.addDetail(self._detail_name,
-            testtools.content.content_from_stream(read_stream, seek_offset=0))
+        self.addDetail(
+            self._detail_name,
+            content_from_stream(read_stream, seek_offset=0))
 
 
 def _byte_stream_factory():
diff --git a/setup.cfg b/setup.cfg
index 0016fa7..21589e9 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -31,7 +31,10 @@ packages =
     fixtures
 
 [extras]
+streams =
+    testtools
 test =
     mock
+    testtools
 docs =
     docutils