BUG: Fix FastParquetImpl.write for non-existent file
`PyArrowImpl` already correctly opens a non-existent file for writing (https://github.com/pandas-dev/pandas/blob/master/pandas/io/parquet.py#L95), with `engine='fastparquet'` this fails for e.g. a GCS URL (though it looks like S3 is already correct):
```
[nav] In [1]: pd.DataFrame().to_parquet('gs://city_data/test/blah.parquet')
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-1-dde78378baaa> in <module>
----> 1 pd.DataFrame().to_parquet('gs://city_data/test/blah.parquet')

~/venvs/model/lib/python3.7/site-packages/pandas/core/frame.py in to_parquet(self, fname, engine, compression, index, partition_cols, **kwargs)
   2215             index=index,
   2216             partition_cols=partition_cols,
-> 2217             **kwargs
   2218         )
   2219

~/venvs/model/lib/python3.7/site-packages/pandas/io/parquet.py in to_parquet(df, path, engine, compression, index, partition_cols, **kwargs)
    250         index=index,
    251         partition_cols=partition_cols,
--> 252         **kwargs
    253     )
    254

~/venvs/model/lib/python3.7/site-packages/pandas/io/parquet.py in write(self, df, path, compression, index, partition_cols, **kwargs)
    171             kwargs["open_with"] = lambda path, _: path
    172         else:
--> 173             path, _, _, _ = get_filepath_or_buffer(path)
    174
    175         with catch_warnings(record=True):

~/venvs/model/lib/python3.7/site-packages/pandas/io/common.py in get_filepath_or_buffer(filepath_or_buffer, encoding, compression, mode)
    212
    213         return gcs.get_filepath_or_buffer(
--> 214             filepath_or_buffer, encoding=encoding, compression=compression, mode=mode
    215         )
    216

~/venvs/model/lib/python3.7/site-packages/pandas/io/gcs.py in get_filepath_or_buffer(filepath_or_buffer, encoding, compression, mode)
     15
     16     fs = gcsfs.GCSFileSystem()
---> 17     filepath_or_buffer = fs.open(filepath_or_buffer, mode)
     18     return filepath_or_buffer, None, compression, True

<decorator-gen-147> in open(self, path, mode, block_size, acl, consistency, metadata)

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in _tracemethod(f, self, *args, **kwargs)
     51         logger.log(logging.DEBUG - 1, tb_io.getvalue())
     52
---> 53     return f(self, *args, **kwargs)
     54
     55

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in open(self, path, mode, block_size, acl, consistency, metadata)
   1148         if 'b' in mode:
   1149             return GCSFile(self, path, mode, block_size, consistency=const,
-> 1150                            metadata=metadata)
   1151         else:
   1152             mode = mode.replace('t', '') + 'b'

<decorator-gen-150> in __init__(self, gcsfs, path, mode, block_size, acl, consistency, metadata)

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in _tracemethod(f, self, *args, **kwargs)
     51         logger.log(logging.DEBUG - 1, tb_io.getvalue())
     52
---> 53     return f(self, *args, **kwargs)
     54
     55

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in __init__(self, gcsfs, path, mode, block_size, acl, consistency, metadata)
   1276             raise NotImplementedError('File mode not supported')
   1277         if mode == 'rb':
-> 1278             self.details = gcsfs.info(path)
   1279             self.size = self.details['size']
   1280         else:

<decorator-gen-136> in info(self, path)

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in _tracemethod(f, self, *args, **kwargs)
     51         logger.log(logging.DEBUG - 1, tb_io.getvalue())
     52
---> 53     return f(self, *args, **kwargs)
     54
     55

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in info(self, path)
    863
    864         try:
--> 865             return self._get_object(path)
    866         except FileNotFoundError:
    867             logger.debug("info FileNotFound at path: %s", path)

<decorator-gen-122> in _get_object(self, path)

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in _tracemethod(f, self, *args, **kwargs)
     51         logger.log(logging.DEBUG - 1, tb_io.getvalue())
     52
---> 53     return f(self, *args, **kwargs)
     54
     55

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in _get_object(self, path)
    539             raise FileNotFoundError(path)
    540
--> 541         result = self._process_object(bucket, self._call('GET', 'b/{}/o/{}', bucket, key).json())
    542
    543         return result

<decorator-gen-121> in _call(self, method, path, *args, **kwargs)

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in _tracemethod(f, self, *args, **kwargs)
     51         logger.log(logging.DEBUG - 1, tb_io.getvalue())
     52
---> 53     return f(self, *args, **kwargs)
     54
     55

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in _call(self, method, path, *args, **kwargs)
    482                 r = self.session.request(method, path,
    483                                          params=kwargs, json=json, headers=headers, data=data, timeout=self.requests_timeout)
--> 484                 validate_response(r, path)
    485                 break
    486             except (HttpError, RequestException, RateLimitException, GoogleAuthError) as e:

~/venvs/model/lib/python3.7/site-packages/gcsfs/core.py in validate_response(r, path)
    156
    157         if r.status_code == 404:
--> 158             raise FileNotFoundError(path)
    159         elif r.status_code == 403:
    160             raise IOError("Forbidden: %s\n%s" % (path, msg))

FileNotFoundError: https://www.googleapis.com/storage/v1/b/city_data/o/test%2Fblah.parquet
```

Can you add a test for the bug this fixes It seems like the only real way to test this is a GCS-specific test like the s3 roundtrip test here: https://github.com/pandas-dev/pandas/blob/master/pandas/tests/io/test_parquet.py#L525-L527

I think everything else besides the GCS branch of the code ends up ignoring the `mode` arg, so the local file version is (accidentally?) correct.

Any other ideas or suggestions on how to set up a test bucket...? I take it nothing in test_gcs is helpful for this?  Can you mock the relevant server-side behavior? @jbrockmendel I took a stab at a test, it doesn't check much but it does fail on master and pass here so 🤷‍♂😄  Oh, whoops, can you add a release notes for 1.0?

Also, you could update the comments under the `is_s3_url` check. Those aren't specific to s3 anymore. > Oh, whoops, can you add a release notes for 1.0?
> 
> Also, you could update the comments under the `is_s3_url` check. Those aren't specific to s3 anymore.

@TomAugspurger done! Linting issue with that last commit :( Can you run black and repush?

Looks like a merge conflict also. oops, still not used to black :) done @bnaul you have a small linting error:

```
Check import format using isort
ERROR: /home/vsts/work/1/s/pandas/tests/io/test_gcs.py Imports are incorrectly sorted.
Check import format using isort DONE
##[error]Bash exited with code '1'.
##[section]Finishing: Linting
``` BTW, for black and isort, I really started to appreciate pre-commit hooks to avoid getting those failures after pushing to github. See https://dev.pandas.io/development/contributing.html#python-pep8-black Thanks!