Closing a connection tries to close already closed cursors

When closing a connection the library will call `.close()` on every cursor created for that connection, including closed ones. While the code works, it produces a lot of logs of error level, polluting the logs.

#### Environment details

  - OS type and version: Mac OS 10.15.7
  - Python version: Python 3.8.2
  - pip version: pip 20.3.3
  - `google-cloud-bigquery` version: 2.7.0

#### Steps to reproduce

  1. Close a cursor
  2. Close the connection
  3. Error log shows `Exception closing connection <google.cloud.bigquery.dbapi.connection.Connection object at 0x...>`

#### Code example

```python
from contextlib import closing

# using pybigquery
with closing(engine.raw_connection()) as conn:
    with closing(conn.cursor()) as cursor:
        cursor.execute(sql)
```

#### Stack trace
```
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 270, in _close_connection
    self._dialect.do_close(connection)
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 549, in do_close
    dbapi_connection.close()
  File "/usr/local/lib/python3.7/site-packages/google/cloud/bigquery/dbapi/_helpers.py", line 258, in with_closed_check
    return method(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/google/cloud/bigquery/dbapi/connection.py", line 79, in close
    cursor_.close()
  File "/usr/local/lib/python3.7/site-packages/google/cloud/bigquery/dbapi/_helpers.py", line 257, in with_closed_check
    raise exc_class(exc_msg)
google.cloud.bigquery.dbapi.exceptions.ProgrammingError: Operating on a closed cursor.
```

#### Suggested fix

```python
# google/cloud/bigquery/dbapi/connection.py
class Connection(object):
    ...
    def close(self):
        ...
        for cursor_ in self._cursors_created:
            if not cursor_._closed:
                cursor_.close()
```

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Closing a connection tries to close already closed cursors #497

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Closing a connection tries to close already closed cursors #497

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