sim.pysim: only import pyvcd when needed.

In some environments (e.g. Pyodide) it may be advantageous to not load
this library, and with the import at file level, it makes the entire
simulator unusable, not just `PySimEngine.write_vcd`.

This might also help people whose Python environments are unusually
broken, whom we've historically accommodated.

amaranth/sim/pysim.py

@@ -1,8 +1,6 @@
 from contextlib import contextmanager
 import itertools
 import re
-from vcd import VCDWriter
-from vcd.gtkw import GTKWSave
 
 from ..hdl import *
 from ..hdl._repr import *
@@ -39,6 +37,10 @@ class _VCDWriter:
             raise NotImplementedError
 
     def __init__(self, fragment, *, vcd_file, gtkw_file=None, traces=()):
+        # Although pyvcd is a mandatory dependency, be resilient and import it as needed, so that
+        # the simulator is still usable if it's not installed for some reason.
+        import vcd, vcd.gtkw
+
         if isinstance(vcd_file, str):
             vcd_file = open(vcd_file, "w")
         if isinstance(gtkw_file, str):
@@ -47,13 +49,13 @@ class _VCDWriter:
         self.vcd_signal_vars = SignalDict()
         self.vcd_memory_vars = {}
         self.vcd_file = vcd_file
-        self.vcd_writer = vcd_file and VCDWriter(self.vcd_file,
+        self.vcd_writer = vcd_file and vcd.VCDWriter(self.vcd_file,
             timescale="1 ps", comment="Generated by Amaranth")
 
         self.gtkw_signal_names = SignalDict()
         self.gtkw_memory_names = {}
         self.gtkw_file = gtkw_file
-        self.gtkw_save = gtkw_file and GTKWSave(self.gtkw_file)
+        self.gtkw_save = gtkw_file and vcd.gtkw.GTKWSave(self.gtkw_file)
 
         self.traces = []