Update tox.ini file

Moves dependencies out to shared test-requirements.txt, except for
pytest-operator, as that patches pytest in ways that take longer to run
and are unnecessary for the unit/lint tests.

test-requirements.txt

@@ -0,0 +1,6 @@
+bcrypt<3.3
+black==20.8b1
+flake8<4.1
+juju<2.10
+pytest<6.3
+pyyaml<6.1

tox.ini

@@ -10,26 +10,19 @@ envlist = lint,unit,integration
 setenv =
     PYTHONPATH={toxinidir}:{toxinidir}/src
     PYTHONBREAKPOINT=ipdb.set_trace
-
-[testenv:unit]
 deps =
-    pyyaml
-    pytest
-    bcrypt
     -r{toxinidir}/requirements.txt
-commands = pytest -v --tb native -s {posargs} {toxinidir}/tests/unit
+    -r{toxinidir}/test-requirements.txt
+
+[testenv:unit]
+commands = pytest -vvs {toxinidir}/tests/unit {posargs}
 
 [testenv:integration]
 deps =
-    juju
-    pytest
     pytest-operator
-commands = pytest -v --tb native --show-capture=no --log-cli-level=INFO -s {posargs} {toxinidir}/tests/integration
+commands = pytest -vvs --log-cli-level=INFO {toxinidir}/tests/integration {posargs}
 
 [testenv:lint]
-deps =
-    black
-    flake8
 commands =
     flake8 {toxinidir}/tests {toxinidir}/src
     black --check --diff {toxinidir}/tests {toxinidir}/src