python: upgrade from .format() to f-strings

They are supported since Python 3.5.

python/src/deltachat/_build.py

@@ -171,7 +171,7 @@ def extract_defines(flags):
             match = defines_re.match(line)
             if match:
                 defines.append(match.group(1))
-    return "\n".join("#define {} ...".format(d) for d in defines)
+    return "\n".join(f"#define {d} ..." for d in defines)
 
 
 def ffibuilder():