Move libjuju pinning to a constraints file

By default libjuju 2.9 will be used, this can overriden by passing the
PIP_CONSTRAINTS environment variable

Examples:

    PIP_CONSTRAINTS=./constraints-juju31.txt tox -e pep8 --recreate

This allows running functional tests with different versions of juju

constraints-juju29.txt

@@ -0,0 +1 @@
+juju>=2.9.0,<3.0.0

constraints-juju31.txt

@@ -0,0 +1 @@
+juju>=3.1.0,<3.2.0

constraints-juju32.txt

@@ -0,0 +1 @@
+juju>=3.2.0,<3.3.0

constraints-juju33.txt

@@ -0,0 +1 @@
+juju>=3.3.0,<3.4.0

test-requirements.txt

@@ -16,5 +16,8 @@ keystoneauth1
 oslo.config
 python-novaclient
 tenacity>8.2.0
+# To force the installation of an specific version of libjuju use a constraints
+# file, e.g.: `env PIP_CONSTRAINTS=./constraints-juju31.txt tox -e func-target`
+juju
 # https://github.com/go-macaroon-bakery/py-macaroon-bakery/issues/94
 macaroonbakery!=1.3.3

tox.ini

@@ -17,23 +17,21 @@ passenv =
   CS_*
   OS_*
   TEST_*
-extras = {env:EXTRA}
+  PIP_*
 deps =
-  -r{toxinidir}/test-requirements.txt
+    -c{env:PIP_CONSTRAINTS:{toxinidir}/constraints-juju29.txt}
+    -r{toxinidir}/test-requirements.txt
 commands = pytest --cov=./zaza/ {posargs} {toxinidir}/unit_tests
 
 [testenv:py3]
 basepython = python3
-deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]
 basepython = python3
-deps = -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} zaza unit_tests
 
 [testenv:venv]
 basepython = python3
-deps = -r{toxinidir}/test-requirements.txt
 commands = /bin/true
 
 [flake8]
@@ -51,7 +49,6 @@ commands = sphinx-build -W -b html -d {toxinidir}/doc/build/doctrees . {toxinidi
 
 [testenv:func]
 basepython = python3
-deps = -r{toxinidir}/test-requirements.txt
 commands =
     {envdir}/bin/python3 setup.py install
     functest-run-suite --keep-faulty-model
@@ -65,14 +62,12 @@ commands =
 
 [testenv:func-target-extended]
 basepython = python3
-deps = -r{toxinidir}/test-requirements.txt
 commands =
     {envdir}/bin/python3 setup.py install
     functest-run-suite --keep-model --test-directory {toxinidir}/tests-extended --log INFO --bundle {posargs}
 
 [testenv:remove-placement]
 basepython = python3
-deps = -r{toxinidir}/test-requirements.txt
 commands =
     {envdir}/bin/python3 setup.py install
     remove-placement {posargs}