Feature/test suite (#14)

* basic tests for categories namespace

* add basic tests for tags api

* add basic tests for sources api namespace

* create basic tests for releases api namespace

* add basic tests for series api namespace

* create test for decorators

* use src for tests

* first full test-suite of basic API functionality, closes issue #8

* new CI for lint and test package

* upgrade black in CI, use fred api key from GH secrets

* reformat files

* update CI only test for 3.9 & 3.10

* only run CI with python 3.10.x

* run ci using coverage

* add coverage upload

* add new job skeleton to CI

* upload to PyPI when publishing new release/prerelease

* update readme

Author: zachspar

README.md

@@ -1,8 +1,10 @@
+[![codecov](https://codecov.io/gh/zachspar/fred-py-api/branch/main/graph/badge.svg?token=BG1948D8Y7)](https://codecov.io/gh/zachspar/fred-py-api)
+
 # Fred Python API
 A fully-featured FRED Python Wrapper.
 
 ## Documentation:
  - [fred-py-api Docs](https://github.com/zachspar/fred-py-api/wiki)
 
 ### FRED References:
- - [API Documentation](https://fred.stlouisfed.org/docs/api/fred/)
+ - [API Documentation](https://fred.stlouisfed.org/docs/api/fred/)