Allow PathLib objects in sources

[mwichmann]

Changes BuilderBase._adjustixes() to try converting the files arguments to path string - in a way that doesn't flip an existing SCons Node back to a string.

Unittest for _adjustixes is added to validate.

Fixes #4398

Open question:

A similar change in .src_builder_sources() may be needed, but the context here allows just using str() if needed, like:

diff --git a/SCons/Builder.py b/SCons/Builder.py
index 16b0cc22d..411bd1d8c 100644
--- a/SCons/Builder.py
+++ b/SCons/Builder.py
@@ -767,6 +767,8 @@ class BuilderBase:

         result = []
         for s in SCons.Util.flatten(source):
+            if isinstance(s, os.PathLike):
+                s = str(s)
             if SCons.Util.is_String(s):
                 match_suffix = match_src_suffix(env.subst(s))
                 if not match_suffix and '.' not in s:

Not sure at the moment how to force something down this code path so can check - comments welcome.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation