pyconvert rule caching is buggy

[LilithHafner]

Affects: PythonCall

Describe the bug
When I add a new conversion rule, it only applies when converting to Julia types T that have never had pyconvert(T, x) called on them.

julia> pyexec("""
       class Hello:
           pass
       """, Main)

julia> x = pyeval("Hello()", Main)
Python: <Hello object at 0x14759a210>

julia> pyconvert(Any, x)
Python: <Hello object at 0x14759a210>

julia> t = pytype(x)
Python: <class 'Hello'>

julia> PythonCall.pyconvert_add_rule(pyconvert(Any, t.__module__)*":"*pyconvert(Any, t.__qualname__), String, (_, _) -> "Hello!!") 

julia> pyconvert(Any, x)
Python: <Hello object at 0x14759a210> ### BUG ### <===========================================

julia> pyconvert(String, x)
"Hello!!"

julia> empty!.(values(PythonCall.PYCONVERT_RULES_CACHE)) ### A possible fix ###
2-element Vector{Dict{Ptr{PythonCall.C.PyObject}, Vector{Function}}}:
 Dict()
 Dict()

julia> pyconvert(Any, x)
"Hello!!"

Your system
PythonCall v0.9.14

Possible fix
Make pyconvert_add_rule clear the rule cache