Pin PyYAML<5.4.0

PyYAML fails to build with Cython, this patch pins PyYAML to the latest
version that doesn't use Cython.

Also relax the pinning of charm-tools to allow the use of maintenance
releases.

Related-Bug: #2030789
Change-Id: I5131e6401fced5b509cadf717553fdfcbba07c0a

requirements.txt

@@ -11,7 +11,8 @@ setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb
 
 # Build requirements
 cffi==1.14.6; python_version < '3.6'  # cffi 1.15.0 drops support for py35.
-charm-tools==2.8.4
+pyyaml>=5.0,<5.4.0  # https://github.com/juju/charm-tools/pull/659
+charm-tools<3.0
 
 simplejson
 

tox.ini

@@ -97,7 +97,8 @@ commands = stestr run --slowest {posargs}
 [testenv:pep8]
 basepython = python3
 deps = flake8==3.9.2
-       charm-tools==2.8.4
+       pyyaml>=5.0,<5.4.0
+       charm-tools<3.0
 commands = flake8 {posargs} src unit_tests
 
 [testenv:func-target]