FIX: set exist_ok=True in os.makedirs()

[Dengda98]

replaces a non-atomic pattern for directory creation:

if not os.path.exists(dirname):
    os.makedirs(dirname)

with the safer:

os.makedirs(dirname, exist_ok=True)

In the parallel state (merged in #110 ), the original pattern is prone to race conditions: two processes may check for directory existence and both attempt to create it at the same time, will case FileExistsError. Using exist_ok=True makes directory creation atomic and thread-safe.