Exercise 3: Two Independent Real Clients, Two Independent Real Sessions — Possible Solution
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THE SETUP
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    jar_a = http.cookiejar.CookieJar()
    opener_a = urllib.request.build_opener(urllib.request.HTTPCookieProcessor(jar_a))

    jar_b = http.cookiejar.CookieJar()
    opener_b = urllib.request.build_opener(urllib.request.HTTPCookieProcessor(jar_b))

Two completely separate real CookieJar instances, each simulating its
own real browser with its own real cookie storage, both talking to
the exact same running App instance.

VERIFIED, REAL RESULT (interleaved on purpose)
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    client A: b'You have visited 1 times'
    client A: b'You have visited 2 times'
    client A: b'You have visited 3 times'
    client B: b'You have visited 1 times'
    client A: b'You have visited 4 times'
    client B: b'You have visited 2 times'

Client A reaches 4 total visits and client B reaches 2, each counting
up correctly on its own, real, independent timeline - regardless of
the two clients' requests being interleaved rather than run one after
the other.

WHY THIS WORKS AS AN ANSWER
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Each opener holds its own real Set-Cookie response and resends its own
real session_id on every later request - opener_a's own cookie and
opener_b's own cookie are two genuinely different strings the whole
time, so SessionStore.load() looks up two entirely separate dict
entries. Interleaving the real requests (A, A, A, B, A, B) rather than
running each client's own requests as one uninterrupted block rules
out any accidental ordering effect - the two counts stay correctly
separate no matter which client's request lands on the server first.