fix trailing slash in paypal webhook route

[nikochiko]

Q/A checklist

• If you add new dependencies, did you update the lock file?

poetry lock --no-update

• Run tests

ulimit -n unlimited && ./scripts/run-tests.sh

• Do a self code review of the changes - Read the diff at least twice.
• Carefully think about the stuff that might break because of this change - this sounds obvious but it's easy to forget to do "Go to references" on each function you're changing and see if it's used in a way you didn't expect.
• The relevant pages still run when you press submit
• The API for those pages still work (API tab)
• The public API interface doesn't change if you didn't want it to (check API tab > docs page)
• Do your UI changes (if applicable) look acceptable on mobile?
• Ensure you have not regressed the import time unless you have a good reason to do so.
  You can visualize this using tuna:

python3 -X importtime -c 'import server' 2> out.log && tuna out.log

To measure import time for a specific library:

$ time python -c 'import pandas'

________________________________________________________
Executed in    1.15 secs    fish           external
   usr time    2.22 secs   86.00 micros    2.22 secs
   sys time    0.72 secs  613.00 micros    0.72 secs

To reduce import times, import libraries that take a long time inside the functions that use them instead of at the top of the file:

def my_function():
    import pandas as pd
    ...

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