Migrate template to Python3

[MikiDi]

This PR aims to migrate the Python template to Python3 + adds various major improvements. Currently marked as draft, allowing for discussion prior to formal review.

Bugfixes

• Escaping strings with triple quotes SPARQL queries
• Removed circular dependency of the "graph" property between web.py and helpers.py
• Make SPARQL_TIMEOUT work as intended
• Make session_id_header and rewrite_url_header work with the right headers

features

• Migrated to Python3 (2.7 EOL since last year)
• Refactored SPARQL escape helper functions
• Refactored "template-consuming service" import logic
• Proper WSGI server for production use
• Reworked and extended documentation
• support for mu-authorization headers

Discussion

• Is a production WSGI-server actually needed? ("We count on other layers of the stack for security. There is little need for concurrent request handling.")
• Linting: This PR includes a .pylintrc configured for developing microservices based on this template. See README for rationale & See discussion here

[MikiDi]

Preliminary conclusions of above mentioned discussion-points:

• Is a production WSGI-server actually needed? ("We count on other layers of the stack for security. There is little need for concurrent request handling.")

Yes, a production server is needed. By default however, we configure the amount of workers by 1, which makes the default setup behave in a similar way as the (current) dev server does. The template consumer can configure the amount of spawned workers to their own liking.

• Linting: This PR includes a .pylintrc configured for developing microservices based on this template. See README for rationale & See discussion here

Providing linting config for a local setup encourages a way of development that goes against the mu.semte.ch-idea that development should happen in a dockerized environment. Despite the fact that the provided linting configuration lowers the bar for novice (mu.semte.ch-) developers, we will not include the linting config in this PR, but rather discuss it separately.