Opening file object in append mode fail.

[RemiDesgrange]

Expected behavior and actual behavior.

Opening a file-descriptor (BytesIO, StringIO, or an openned file doesn't work with the append mode.

This is normal see implementation here: https://github.com/Toblerity/Fiona/blob/master/fiona/__init__.py#L212 and https://github.com/Toblerity/Fiona/blob/master/fiona/__init__.py#L212

Steps to reproduce the problem.

import fiona
from io import BytesIO

fp = BytesIO(b"some random valid geopackage bytes")
with fiona.open(fp, 'a', driver="Gpkg", schema={"something": "valid"}) as gpkg_file:
    pass

Operating system

Linux (Arch)

Fiona and GDAL version and provenance

1.8.20 manylinux1 wheel installed from PyPI using pip version 21.1.3.

GDAL 3.3, installed via package manager.

[jsta]

I'm seeing an error message:

with fiona.open(fp, 'a', driver="Gpkg", schema={"something": "valid"}) as gpkg_file:
  File "/home/jsta/.conda/envs/vote/lib/python3.8/site-packages/fiona/env.py", line 417, in wrapper
    return f(*args, **kwargs)
  File "/home/jsta/.conda/envs/vote/lib/python3.8/site-packages/fiona/__init__.py", line 253, in open
    path = parse_path(fp)
  File "/home/jsta/.conda/envs/vote/lib/python3.8/site-packages/fiona/path.py", line 132, in parse_path
    elif path.startswith('/vsi'):
AttributeError: '_io.BytesIO' object has no attribute 'startswith

[sgillies]

@jsta we only support read-only and write-only access to datasets in a Python file object. I'm making this more clear in 55d039e.