Rewrite of pyairtable.formulas

[mesozoic]

This is a breaking change to the pyairtable.formulas module, so it would have to be released as 3.0, but it introduces a more thorough way for library users to construct formulas and chain them together.

This module allows for complex nested expressions that are built up through chaining; for example:

>>> formula = match({"Customer": "Alice"})
>>> formula
EQ(Field('Customer'), 'Alice')
>>> complex = formula & Field('Delivery Date').gte(TODAY())
>>> complex
AND(EQ(Field('Customer'), 'Alice'), GTE(Field('Delivery Date'), TODAY()))
>>> str(complex)
"AND({Customer}='Alice', {Delivery Date}>=TODAY())"

The way all of the functions are exposed is a bit janky, and relies on code generation running against a text file that must be refreshed whenever Airtable releases new functions. I'll look to incorporate the approach laid out in #328 for reading those from the formula playground instead.

A summary of changes:

• formula= kwargs now accept either str or an instance of Formula
• The new Formula class has subclasses Comparison and FunctionCall
• The match() function signature remains the same, though it returns a Formula class instead of str
• to_airtable_value() is removed, and replaced with a similar (but not same) to_formula_str()
• field_name() replaces the FIELD() function
• EQ, NE, GT, GTE, LT, LTE replace equivalent functions (EQUAL(), etc.)
• Functions like AND() and OR() now return Comparison (a subclass of Formula) instead of str
• All Airtable formula functions are exposed, and return FunctionCall (a subclass of Formula)
• ORM fields descriptors contain .eq(), etc. convenience methods for producing formulas.

Resolves #304
Resolves #305
Resolves #324
Resolves #328

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.95%. Comparing base (b676bf0) to head (be844ec).

❗ Current head be844ec differs from pull request most recent head 2744c67. Consider uploading reports for the commit 2744c67 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #329      +/-   ##
==========================================
+ Coverage   99.60%   99.95%   +0.35%     
==========================================
  Files          25       25              
  Lines        2025     2307     +282     
==========================================
+ Hits         2017     2306     +289     
+ Misses          8        1       -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.