OverflowError: date value out of range

* **asyncpg version**: 0.18.3
* **PostgreSQL version**: 10.7
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: Yes
* **Python version**: 3.7
* **Platform**: Mac OS
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Yes
* **If you built asyncpg locally, which version of Cython did you use?**:
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**:

asyncpg is failing to parse timestamp

code:
```
import asyncio
import asyncpg

async def run():
conn = await asyncpg.connect(user='secure', password='secure',
database='secure',
host='secure.com', port=15432, timeout=600)

values = await conn.fetch('''SELECT _ts FROM Events''')
print(values)
await conn.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(run())
```

```
Timestamp format: 2019-01-01T08:42:25.921Z
```

```
Traceback (most recent call last):
File "/scratch_65.py", line 21, in
loop.run_until_complete(run())
File "/usr/local/Cellar/python/3.7.2/Frameworks/Python.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py", line 584, in run_until_complete
return future.result()
File "/scratch_65.py", line 15, in run
values = await conn.fetch('''SELECT _ts FROM Events''')
File "/lib/python3.7/site-packages/asyncpg/connection.py", line 421, in fetch
return await self._execute(query, args, 0, timeout)
File "/lib/python3.7/site-packages/asyncpg/connection.py", line 1414, in _execute
query, args, limit, timeout, return_status=return_status)
File "/lib/python3.7/site-packages/asyncpg/connection.py", line 1422, in __execute
return await self._do_execute(query, executor, timeout)
File "/lib/python3.7/site-packages/asyncpg/connection.py", line 1444, in _do_execute
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 252, in asyncpg.pgproto.pgproto.timestamptz_decode
OverflowError: date value out of range

```

Start with asyncpg/pgproto/./codecs/datetime.pyx at timestamptz_decode, then reproduce the failure using the provided Python query against PostgreSQL. Trace how the reported timestamp is decoded and add coverage for this case if the repository has a relevant test location. Done means the query returns the timestamp without OverflowError.

Written by the indexing model from the issue text.