Make Image.gv_dir an InitVar

It is only used during initialization in the __post_init__() function
and does need to be a proper attribute.
https://docs.python.org/3/library/dataclasses.html#init-only-variables

src/wireviz/DataClasses.py

@@ -2,15 +2,15 @@
 # -*- coding: utf-8 -*-
 
 from typing import Optional, List, Any, Union
-from dataclasses import dataclass, field
+from dataclasses import dataclass, field, InitVar
 from pathlib import Path
 from wireviz.wv_helper import int2tuple, aspect_ratio
 from wireviz import wv_colors
 
 
 @dataclass
 class Image:
- gv_dir: Path # Directory of .gv file injected as context during parsing
+ gv_dir: InitVar[Path] # Directory of .gv file injected as context during parsing
  # Attributes of the image object <img>:
  src: str
  scale: Optional[str] = None # false | true | width | height | both
@@ -22,7 +22,7 @@ class Image:
  caption: Optional[str] = None
  # See also HTML doc at https://graphviz.org/doc/info/shapes.html#html
 
- def __post_init__(self):
+ def __post_init__(self, gv_dir):
 
  if self.fixedsize is None:
  # Avoid default True if the user specify a self.scale value.
@@ -38,10 +38,10 @@ def __post_init__(self):
  # because Graphviz requires both when fixedsize=True.
  if self.height:
  if not self.width:
- self.width = self.height * aspect_ratio(self.gv_dir.joinpath(self.src))
+ self.width = self.height * aspect_ratio(gv_dir.joinpath(self.src))
  else:
  if self.width:
- self.height = self.width / aspect_ratio(self.gv_dir.joinpath(self.src))
+ self.height = self.width / aspect_ratio(gv_dir.joinpath(self.src))
 
 
 @dataclass