move attic test dependency into own file

so you can just empty that file to remove the attic-based tests
when testing in a OpenSSL 1.1 environment.

requirements.d/attic.txt

@@ -0,0 +1,5 @@
+# Please note:
+# attic only builds using OpenSSL 1.0.x, it can not be installed using OpenSSL >= 1.1.0.
+# If attic is not installed, our unit tests will just skip the tests that require attic.
+attic
+

tox.ini

@@ -10,7 +10,7 @@ envlist = py{34,35},flake8
 changedir = {toxworkdir}
 deps =
      -rrequirements.d/development.txt
-     attic
+     -rrequirements.d/attic.txt
 commands = py.test --cov=borg --cov-config=../.coveragerc --benchmark-skip --pyargs {posargs:borg.testsuite}
 # fakeroot -u needs some env vars:
 passenv = *