Feature/issue 83 improve test configuration

[bayesflows]

• Delete Travis CI files
• Migration to modern pytest with GitHub Actions for automated testing on every PR/commit.
• Creation of an initial GitHub Actions workflow and minimal pytest configuration to ensure full backward compatibility with existing tests in .github/workflows/tests.yml.
• Creation of docs relative to this migration of tests in docs folder
• Exclude CI tests for Python 3.7 and 3.8 because of incompatibility of type hint syntax for these versions (Error: TypeError: 'type' object is not subscriptable)