Remove astpretty as a dependency of pyflamegpu

astpretty is only used for prettyprint debugging in codegen testing - it is not a pyflamegpu dependency.

This commit also makes it an optional test_codegen.py dependency (but installs it in the test venv by default)

Closses #1164

tests/python/codegen/test_codegen.py

@@ -9,7 +9,8 @@
 try:
     import astpretty
 except ImportError:
-    pass
+    print("Install astpretty if ast tree printing required for test debugging")
+
 
 DEBUG_OUT = True
 EXCEPTION_MSG_CHECKING = True
@@ -596,7 +597,7 @@ def _checkExpected(self, source, expected):
             try:
                 astpretty.pprint(tree)
             except NameError:
-                print("Install astpretty if debugging required")
+                pass
         code = pyflamegpu.codegen.codegen(tree)
         # remove new lines
         code = code.strip()