BC dates result in error running the query

* **asyncpg version**: asyncpg=0.18.3=py37h1de35cc_0
* **PostgreSQL version**: PostgreSQL 11.1 on x86_64-apple-darwin18.2.0, compiled by Apple LLVM version 10.0.0 (clang-1000.11.45.5), 64-bit
* **Do you use a PostgreSQL SaaS? No
* **Python version**: 3.7
* **Platform**: osx
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Yes
* **If you built asyncpg locally, which version of Cython did you use?**: No
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: Tried only asyncio

When a date or datetime column includes dates in the form of `0001-01-01 BC` which are valid postgres dates, the driver returns the following:

```
result = await executor(stmt, None)
File "asyncpg/protocol/protocol.pyx", line 196, in bind_execute
File "asyncpg/protocol/coreproto.pyx", line 65, in asyncpg.protocol.protocol.CoreProtocol._read_server_messages
File "asyncpg/protocol/coreproto.pyx", line 202, in asyncpg.protocol.protocol.CoreProtocol._process__bind_execute
File "asyncpg/protocol/coreproto.pyx", line 498, in asyncpg.protocol.protocol.CoreProtocol._parse_data_msgs
File "asyncpg/protocol/protocol.pyx", line 754, in asyncpg.protocol.protocol.BaseProtocol._decode_row
File "asyncpg/protocol/prepared_stmt.pyx", line 282, in asyncpg.protocol.protocol.PreparedStatementState._decode_row
File "asyncpg/protocol/codecs/base.pyx", line 288, in asyncpg.protocol.protocol.Codec.decode
File "asyncpg/protocol/codecs/base.pyx", line 195, in asyncpg.protocol.protocol.Codec.decode_scalar
File "asyncpg/pgproto/./codecs/datetime.pyx", line 133, in asyncpg.pgproto.pgproto.date_decode
ValueError: ordinal must be >= 1
```

**ValueError: ordinal must be >= 1**

I assume this is some kind of a problem mapping from the BC date to a python date, however this is annoying since valid postgres tables can't be queries. At least in the interim it should be possible to return None if for some reason the date couldn't be parsed but not give up entirely. Is there a quick and dirty way around that?

Start in asyncpg/pgproto/./codecs/datetime.pyx at the date_decode path shown in the traceback, and reproduce the failure with a PostgreSQL date or datetime containing 0001-01-01 BC. Check the existing date codec tests and determine the supported result for BC values; done means decoding such a row no longer aborts the query and the behavior is covered by a regression test.

Written by the indexing model from the issue text.