tests: update requirements

cheroot release(/"changes"?) notes:
    #218 via PR #219: Fix HTTP parser to return 400 on invalid major-only HTTP version in Request-Line.
    #198 via 9f7affe: Fix race condition when toggling stats counting in the middle of request processing.
    Improve post Python 3.9 compatibility checks.
    Fix support of abstract namespace sockets.
    #222 via 621f4ee: Fix socket.SO_PEERCRED constant fallback value under PowerPC
    Revisit PR #85 under PR #221. Now backports.functools_lru_cache is only required on Python 3.2 and earlier.
    CherryPy #1206 via PR #204: Fix race condition in threadpool shrink code.
    PR #224: Refactored “open URL” behavior in webtest to rely on retry_call. Callers can no longer pass raise_subcls or ssl_context positionally, but must pass them as keyword arguments.
    #231 via PR #232: Remove custom setup.cfg parser handling, allowing the project (including sdist) to build/run on setuptools 41.4. Now building cheroot requires setuptools 30.3 or later (for declarative config support) and preferably 34.4 or later (as indicated in pyproject.toml).
    Workers are now request-based, addressing the long-standing issue with keep-alive connections (#91 via PR #199).
    Deprecated use of negative timeouts as alias for infinite timeouts in ThreadPool.stop.
    CherryPy #1662 via PR #74: For OPTION requests, bypass URI as path if it does not appear absolute.
    CherryPy #1818: Restore support for None default argument to WebCase.getPage().

https://github.com/cherrypy/cheroot/blob/master/CHANGES.rst

flaky changes:
    Bugfixes - Reraise KeyboardInterrupt when running tests under pytest.

https://github.com/box/flaky/blob/v3.6.1/HISTORY.rst#361-2019-08-06

python-bitcoinlib:
    New RPC `generatetoaddress(self,numblocks,addr)`.
    Fixed Python 2.7 incompatibility.
    Various OpenSSL fixes, including a memory leak.

https://github.com/petertodd/python-bitcoinlib/blob/python-bitcoinlib-v0.10.2/release-notes.md#v0102

pytest release notes:
    A lot of misc fixes, see https://docs.pytest.org/en/latest/changelog.html.

tests/requirements.txt

@@ -1,13 +1,13 @@
 flake8==3.7.8
-pytest==5.0.1
+pytest==5.3.1
 Flask==1.1.1
-cheroot==6.5.5
+cheroot==8.2.1
 ephemeral-port-reserve==1.1.1
-flaky==3.6.0
+flaky==3.6.1
 pytest-benchmark==3.2.2
 pytest-forked==1.0.2
 pytest-xdist==1.29.0
-python-bitcoinlib==0.10.1
+python-bitcoinlib==0.10.2
 tqdm==4.32.2
 pytest-timeout==1.3.3
 psycopg2-binary==2.8.3