Fix Connection class _copy_in private method

[ABCDeath]

I've found an issue in Connection.copy_to_table() method with a path-like source argument. The following code throws exception io.UnsupportedOperation: read:

import asyncio
import asyncpg

async def main():
    conn = await asyncpg.connect(connection_string)
    await conn.execute('CREATE TABLE test_table(a text, "b~" text, i int);')
    await conn.copy_to_table('test_table', source=filename)
    
filename = 'table_content'
f = open(filename, 'wb')
f.write(
    '\n'.join([
        'a1\tb1\t1',
        'a2\tb2\t2',
        'a3\tb3\t3',
        'a4\tb4\t4',
        'a5\tb5\t5',
        ''
    ]).encode('utf-8')
)
f.close()

asyncio.run(main())

It looks like Connection._copy_in() method opens file in a wrong mode (wb instead of rb).
So this fix:

• Change file opening mode to rb;
• Add test case for Connection.copy_to_table() method with a path-like source.

[elprans]

Good catch! Thanks!