Warn and force --nostrip on the presence of f-strings in waf files

wscript

@@ -116,7 +116,7 @@ def options(opt):
 	opt.add_option('--namesfrom', action='store', help='Obtain the file names from a model archive', dest='namesfrom', default=None)
 	opt.load('python')
 
-def process_tokens(tokens):
+def process_tokens(tokens, filename):
 	accu = []
 	prev = tokenize.NEWLINE
 
@@ -147,7 +147,10 @@ def process_tokens(tokens):
 			line_buf.append(token)
 		elif type == tokenize.STRING:
 			if not line_buf and token.startswith('"'): pass
-			else: line_buf.append(token)
+			else:
+				if token.lower().startswith('f'):
+					raise ValueError('Found f-strings in %s which require Python >= 3.6, use "waf-light --nostrip"' % filename)
+				line_buf.append(token)
 		elif type == tokenize.COMMENT:
 			pass
 		elif type == tokenize.OP:
@@ -189,10 +192,10 @@ def sfilter(path):
 				with open(path, 'rb') as f:
 					tk = tokenize.tokenize(f.readline)
 					next(tk) # the first one is always tokenize.ENCODING for Python 3, ignore it
-					cnt = process_tokens(tk)
+					cnt = process_tokens(tk, path)
 			else:
 				with open(path, 'r') as f:
-					cnt = process_tokens(tokenize.generate_tokens(f.readline))
+					cnt = process_tokens(tokenize.generate_tokens(f.readline), path)
 		else:
 			with open(path, 'r') as f:
 				cnt = f.read()