Fix for Error fetching deposits

[kabeleh]

Description

Fix for Error fetching deposits: 400 Client Error: BAD REQUEST for url: https://zenodo.org/api/records?q=title%3A%22unimpeded%3A%22&size=1000&page=1

1. size is changed to 25, since this is the allowed maximum (server-side).
2. zenodo blocks requests without user agent (to block bot access). To avoid this, pass a user agent.

Fixes #24

Checklist:

• I have performed a self-review of my own code
• My code is PEP8 compliant (flake8 unimpeded tests)
• My code contains compliant docstrings (pydocstyle --convention=numpy unimpeded)
• New and existing unit tests pass locally with my changes (python -m pytest)
• I have added tests that prove my fix is effective or that my feature works
• I have appropriately incremented the semantic version number in both README.rst and unimpeded/_version.py