--------
kirr:

Extract from https://github.com/zopefoundation/ZEO/pull/195 bits that
add documentation to existing code without changing semantic, and fix
typos.

The only things I added myself with further help from @d-maurer are:

- documentation for server_sync in ClientStorage;
- stub documentation for credentials in ClientStorage.

Even though we agree to deprecate credentials in favour of peer-to-peer
TLS, removing their support should go as a separate step.

For server_sync feature, that
https://github.com/zopefoundation/ZEO/pull/195 currently removes, we
actually do use it for correctness:

https://lab.nexedi.com/nexedi/erp5/blob/eaae74a082a0/product/ERP5Type/tests/custom_zodb.py#L175-179
nexedi/erp5@c663257f
https://github.com/zopefoundation/ZODB/commit/9821696f584f

So document it with the intent to preserve it.

/reviewed-on https://github.com/zopefoundation/ZEO/pull/202

We can use stdlib random module and manually adjust randint to behave
exactly as before Python3, so that testdata are matched exactly as before.

Also drop unnecessary `random.seed(42)` in cache_run. It is not needed
there because random is not used in cache_run at all.

Based on work of @d-maurer in https://github.com/zopefoundation/ZEO/pull/195 .

Reduce ZEO testing time by dropping tests with some storages

Drop ZEO tests with storages `DemoStorage`, `FileStorage` and `HexFileStorage`; `FileStorage` is still tested in the variants with server side blobs and with shared blobs

Full linting with flake8

Co-authored-by: Dieter Maurer <d-maurer@users.noreply.github.com>

Co-authored-by: Dieter Maurer <d-maurer@users.noreply.github.com>

Configure with zope.meta.config.config-package

Python 3.10 support

tpc_vote/abort + log message enhancements

Remove testing for mtacceptor and dead Python versions

Python 3.10 distinguishes SSL contexts for server and client use - and SSL fails with the wrong context type

#165 more accurate asyncio.CancelledError handling

From https://github.com/zopefoundation/ZEO/pull/182#issuecomment-826630195 :

For `protocol.fut('lastTransaction')` vs `ClientDisconnected` in verify - I
would suggest to do the following instead: just move up the try/except that is
_already_ covering the code in verify a bit up - to also cover that
lastTransaction call. This try / except already catches `except Exception` and
does `self.register_failed(protocol, exc)` on a failure:

https://github.com/zopefoundation/ZEO/blob/1070adcbd17b83b9515c43b48ca38687783793e4/src/ZEO/asyncio/client.py#L488-L535

So if we just move its beginning `try` a bit up in here to also cover
`lastTransaction` call here:

https://github.com/zopefoundation/ZEO/blob/1070adcbd17b83b9515c43b48ca38687783793e4/src/ZEO/asyncio/client.py#L485-L488

it will work out automatically. It will work correctly because
`ClientDisconnected` is inheriting as

    ClientDisconnected -> ClientStorageError -> StorageError -> POSError -> Exception

It is also more logical to wrap the whole operation in verify in such a
try/except block, so that if _anything_ at _any_ point fails, we mark the whole
operation as failed.