Stable build fixes

Switch to using the latest charm-tools <3.0.0 to fix two build errors:
- An error related to 'pytz' fixed in charm-tools 2.8.6:
  juju/charm-tools#654
- An error related to ruamel.yaml fixed in charm-tools 2.8.8:
  here: juju/charm-tools#668

Additionally merge bindep.txt to install libpq-dev required for
psycopg2-binary as wheels no longer exist for py36.

Change-Id: Idc16b84dc3dcdf001c73c7d2fdd57b1d08ebfb23

bindep.txt

@@ -0,0 +1 @@
+libpq-dev [platform:dpkg]

requirements.txt

@@ -11,7 +11,7 @@ setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb
 # Build requirements
 pyparsing<3.0.0  # cffi needs pyparsing < 3.0.0.
 cffi==1.14.6; python_version < '3.6'  # cffi 1.15.0 drops support for py35.
-charm-tools==2.8.3
+charm-tools<3.0.0
 
 simplejson
 

tox.ini

@@ -97,7 +97,8 @@ commands = stestr run --slowest {posargs}
 [testenv:pep8]
 basepython = python3
 deps = flake8==3.9.2
-       charm-tools==2.8.3
+       PyYAML==6.0.1
+       charm-tools<3.0.0
 commands = flake8 {posargs} src unit_tests
 
 [testenv:func-target]