Removing Upper Constraint

There is no reason to believe that future versions of PyYAML will
not be compatible with this package, so we are removing this
is an upper bound

Signed-off-by: Mark Beierl <mark.beierl@canonical.com>

Author: Mark Beierl

setup.py

@@ -33,7 +33,7 @@
     install_requires=[
         'macaroonbakery>=1.1,<2.0',
         'pyRFC3339>=1.0,<2.0',
-        'pyyaml>=5.1.2,<=6.0',
+        'pyyaml>=5.1.2',
         'theblues>=0.5.1,<1.0',
         'websockets>=7.0,<8.0 ; python_version<"3.9"',
         'websockets>=8.0,<9.0 ; python_version=="3.9"',