Fixed matplotlib plot style (#25)

MatplotlibDeprecationWarning: The seaborn styles shipped by Matplotlib are deprecated since 3.6, as they no longer correspond to the styles shipped by seaborn. However, they will remain available as 'seaborn-v0_8-<style>'

Fixes
#581

Co-authored-by: Michele Fortunato <mikyluck92@hotmail.it>
Co-authored-by: Robert Martin <11792905+robertmartin8@users.noreply.github.com>

pypfopt/plotting.py

@@ -17,7 +17,7 @@
 from . import CLA, EfficientFrontier, exceptions, risk_models
 
 try:
-    plt.style.use("seaborn-deep")
+    plt.style.use("seaborn-v0_8-deep")
 except Exception:  # pragma: no cover
     pass