meson: fix compatibility with Python 3.7

[8/2759] Generating src/basic/filesystem-sets.c with a custom command (wrapped by meson to capture output)
FAILED: src/basic/filesystem-sets.c
/mnt/work/.local/bin/meson --internal exe --capture src/basic/filesystem-sets.c -- /mnt/work/src/systemd/upstream-fork/main/src/basic/filesystem-sets.py fs-type-to-string filesystem-sets fs-in-group
--- stderr ---
Traceback (most recent call last):
  File "/mnt/work/src/systemd/upstream-fork/main/src/basic/filesystem-sets.py", line 372, in <module>
    generate_fs_in_group()
  File "/mnt/work/src/systemd/upstream-fork/main/src/basic/filesystem-sets.py", line 311, in generate_fs_in_group
    start=[])))
TypeError: sum() takes no keyword arguments

src/basic/filesystem-sets.py

@@ -307,8 +307,7 @@ def generate_fs_in_group():
     print('        switch (fs_group) {')
 
     for name, _, *filesystems in FILESYSTEM_SETS:
-        magics = sorted(set(sum((NAME_TO_MAGIC[fs] for fs in filesystems),
-                                start=[])))
+        magics = sorted(set(sum((NAME_TO_MAGIC[fs] for fs in filesystems), [])))
         enum = 'FILESYSTEM_SET_' + name[1:].upper().replace('-', '_')
         print(f'        case {enum}:')
         opts = '\n                    || '.join(f'F_TYPE_EQUAL(st->f_type, {magic})'
@@ -355,7 +354,7 @@ def magic_defines():
 
 def check():
     kernel_magics = set(magic_defines())
-    our_magics = set(sum(NAME_TO_MAGIC.values(), start=[]))
+    our_magics = set(sum(NAME_TO_MAGIC.values(), []))
     extra = kernel_magics - our_magics
     if extra:
         sys.exit(f"kernel knows additional filesystem magics: {', '.join(sorted(extra))}")