fix: lazy imports for matplotlib & gdstk (#161)

only require matplotlib & gdstk for functionality where we actually use them

contributed by @gerardvt

doc_utils.py

@@ -5,7 +5,6 @@ import subprocess
 from typing import Optional
 
 import chevron
-import matplotlib as mpl
 
 from config import Config, DatasheetConfig
 
@@ -29,6 +28,8 @@ class DocsHelper:
         """
         Format the clock with engineering notation
         """
+        import matplotlib as mpl
+
         if clock == 0:
             return "No Clock"
         else:

render_utils.py

@@ -5,7 +5,6 @@ import re
 import subprocess
 
 import cairosvg  # type: ignore
-import gdstk  # type: ignore
 
 
 # Custom exception for convert_svg_to_png()
@@ -35,6 +34,8 @@ def render_svg(
     filter_layers=None,
     scramble_cells=None,
 ):
+    import gdstk  # type: ignore
+
     library = gdstk.read_gds(gds)
     top_cells = library.top_level()
     assert len(top_cells) == 1