refactor: simplify OrderedDict arguments in lexer

[plamut]

Closes #599. 🦕

Python 3.6+ guarantees that kwargs order is preserved, thus we don't need to assure the order by passing the arguments as a list of tuples (we don't support Python 3.5 anymore). Results in a more compact and cleaner source (IMO).

There are still some OrderedDict instances in tests that could benefit from the same, but I'm not touching that to not cause merge conflicts with the tests refactoring changes @jimfulton is making right now.

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[plamut]

Note - we still need to use OrderedDict instad of a regular dict, because the regular dict's key insertion order should be considered an implementation detail of CPython 3.6 (that behavior has only been promoted to a language spec in Python 3.7).

[tswast]

Ah! I didn't realize kwargs became ordered a version earlier :-)

[plamut]

@jimfulton Assigned to you if you have free cycles and want to get more familiar with the BigQuery code.

[tswast]

Python 3.6+ guarantees that kwargs order is preserved

This is true for CPython, but I recall that the language standard only makes this the case for Python 3.7+. Not that we support/test with alternative runtimes... Maybe worth bringing up in the Python meeting today.

[tswast]

Much cleaner!