Updates to pin charm-tools to 2.8.3

* Update requirements.txt to pin charm-tools to 2.8.3
* Update tox.ini to change the build parameters.
* This upgrades from <2.7 which adds reproducible charms
* Bug was fixed that controlled the default output directory
  doesn't get 'builds' appended.  The tox change puts it
  back so that stable & master both build to the same
  directory.  This may be reviewed in the future.

Change-Id: I238d5c89afddb97f57515e76c33005b31f153787

requirements.txt

@@ -9,7 +9,7 @@
 setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb244e9b27bc56750c44b7c85
 
 # Build requirements
-charm-tools>=2.4.4,<2.8
+charm-tools==2.8.3
 
 # Workaround until https://github.com/juju/charm-tools/pull/589 gets
 # published

tox.ini

@@ -28,7 +28,7 @@ deps =
 [testenv:build]
 basepython = python3
 commands =
-    charm-build --log-level DEBUG -o {toxinidir}/build src {posargs}
+    charm-build --log-level DEBUG -o {toxinidir}/build/builds src {posargs}
 
 [testenv:py3]
 basepython = python3