Otter-grader

* Changed granularity of results to be test case-by-test case rather than by file
* Added ability to list requirements directly in assignment metadata w/out requirements.txt file
* Unified assignment grading workflow and converted local grading to container-per-submission
* Exposed grading internals for non-containerized grading via `otter run` and `otter.api`
* Added plugins for altering grades and execution, incl. built-in plugins
* Added ignorable cells to Otter Assign
* Added `autograder_files` configuration for Otter Assign
* Added passdown of assignment configurations to Otter Generate from Otter Assign
* Fixed whitespace bug in Assign solution parsing
* Resolved conflicts with `nbconvert>=6.0.0`, removed version pin
* Added `otter.assign.utils.patch_copytree` as a patch for `shutil.copytee` on WSL
* Refactored Otter Generate to use `zipfile` to generate zips
* Refactored CLI to allow creation of programmatic API
* Changed `otter generate autograder` to `otter generate`
* Removed `otter generate token` as all interaction with `otter.generate.token.APIClient` can be handled elsewhere
* Added intercell seeding for R Jupyter Notebooks
* Added `ValueError` on unexpected config in `otter.assign.assignment.Assignment`
* Added `--username`, `--password` flags to Otter Assign and Otter Generate
* Added support for Python files
* Removed `FutureWarning` for deprecated global `hidden` key of OK tests
* Add missing file specifier in environment template

* Fixed `ZeroDivisionError` when an assignment has 0 points total

* Fixed error in parsing requirements when using Otter Grade

* Fixed `KeyError` when kernelspec unparsable from notebook in Otter Assign

* Changed Rmd code prompt to `NULL YOUR CODE HERE` for assignment statements and ` YOUR CODE HERE` for whole-line and block removal

* Made requirements specification always throw an error if a user-specified path is not found
* Pinned `nbconvert<6.0.0` as a temporary measure due to new templating issues in that release