Update tox.ini file

Add the 'build' rule and also update the needed dependencies
for tests to properly run.

tox.ini

@@ -14,6 +14,8 @@ skipsdist = True
 sitepackages = False
 # NOTE: Avoid false positives by not skipping missing interpreters.
 skip_missing_interpreters = False
+requires = pip < 20.3
+           virtualenv < 20.0
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
@@ -25,6 +27,7 @@ commands = stestr run --slowest {posargs}
 whitelist_externals = juju
 passenv = HOME TERM CS_* OS_* TEST_*
 deps = -r{toxinidir}/test-requirements.txt
+       -r{toxinidir}/requirements.txt
 
 [testenv:py35]
 basepython = python3.5
@@ -55,7 +58,7 @@ deps = -r{toxinidir}/requirements.txt
 basepython = python3
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = flake8 {posargs} hooks unit_tests tests actions lib files
+commands = flake8 {posargs} unit_tests src
            charm-proof
 
 [testenv:cover]
@@ -118,3 +121,8 @@ commands =
 [flake8]
 ignore = E402,E226
 exclude = */charmhelpers
+
+[testenv:build]
+basepython = python3
+commands =
+    charm-build --log-level DEBUG -o {toxinidir}/build src {posargs}