PR: [2](https://github.com/alphagov/digitalmarketplace-test-utils/pull/8)
Change fixtures.py api
* Remove `valid_pdf`, `valid_jpg` and `valid_jpeg`
* Replace with `valid_pdf_bytes`, `valid_jpeg_bytes` and `valid_jpg_bytes`
The old `BytesIO` objects were mutable file like objects, not suitable for reuse in tests.
New fixtures should be employed in the creation of a new file-like object per usage.
Old code:
client.post('/pdf-document-upload', data={'pdf-document': (valid_pdf, 'test.pdf')})
New code:
client.post('/pdf-document-upload', data={'pdf-document': (BytesIO(valid_pdf_bytes), 'test.pdf')})