Promote PyYaml to a direct dependency

After installing in a fresh Python virtual environment, `tmuxp` fails
with `ModuleNotFoundError: No module named 'yaml'`.
This happens because since switching from `kaptan` to `ConfigReader`,
`PyYaml` is no longer pulled in as a transitive dependency.
Adding `PyYaml` as a direct "main" dependency should fix this.

Author: heindsight

poetry.lock

@@ -49,6 +49,7 @@ python = "^3.7"
 click = "~8"
 libtmux = "~0.15.8"
 colorama = ">=0.3.9"
+PyYAML = "^6.0"
 
 [tool.poetry.dev-dependencies]
 ### Docs ###