Fix some code quality issues

[pnijhara]

• Remove assert statement from non-test files
• Remove unnecessary comprehension
• Use literal syntax instead of function calls to create the data structure

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Find the other issues found here - https://deepsource.io/gh/pnijhara/gitsome/issues/?category=all

This PR also adds .deepsource.toml configuration file to run DeepSource analysis on the repo with. Upon enabling DeepSource, the analysis will run on every PR and commit to detect 560+ types of issues in the changes — including bug risks, anti-patterns, security vulnerabilities, etc.

To enable DeepSource analysis after merging this PR, please follow these steps:

1. Signup on DeepSource with your GitHub account and grant access to this repo.
2. Activate analysis on this repo here.

You can also look at the docs for more details. Do let me know if I can be of any help!