Add support for unions to our Python builtins type system

[charliermarsh]

Summary

Fixes some TODOs introduced in #6538. In short, given an expression like 1 if x > 0 else "Hello, world!", we now return a union type that says the expression can resolve to either an int or a str. The system remains very limited, it only works for obvious primitive types, and there's no attempt to do inference on any more complex variables. (If any expression yields Unknown or TypeError, we propagate that result throughout and abort on the client's end.)

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no changes.

Benchmark

Linux

group                                      main                                   pr
-----                                      ----                                   --
formatter/large/dataset.py                 1.02     10.3±0.04ms     3.9 MB/sec    1.00     10.2±0.04ms     4.0 MB/sec
formatter/numpy/ctypeslib.py               1.03      2.1±0.07ms     8.0 MB/sec    1.00      2.0±0.04ms     8.2 MB/sec
formatter/numpy/globals.py                 1.06    244.1±7.55µs    12.1 MB/sec    1.00    230.2±5.90µs    12.8 MB/sec
formatter/pydantic/types.py                1.02      4.4±0.13ms     5.8 MB/sec    1.00      4.3±0.09ms     5.9 MB/sec
linter/all-rules/large/dataset.py          1.00     13.0±0.07ms     3.1 MB/sec    1.00     12.9±0.06ms     3.1 MB/sec
linter/all-rules/numpy/ctypeslib.py        1.00      3.5±0.01ms     4.8 MB/sec    1.00      3.5±0.04ms     4.8 MB/sec
linter/all-rules/numpy/globals.py          1.00    494.7±1.18µs     6.0 MB/sec    1.00    495.1±4.47µs     6.0 MB/sec
linter/all-rules/pydantic/types.py         1.00      6.7±0.02ms     3.8 MB/sec    1.00      6.7±0.05ms     3.8 MB/sec
linter/default-rules/large/dataset.py      1.00      6.8±0.02ms     6.0 MB/sec    1.00      6.8±0.02ms     6.0 MB/sec
linter/default-rules/numpy/ctypeslib.py    1.00   1500.0±8.76µs    11.1 MB/sec    1.00  1504.1±15.43µs    11.1 MB/sec
linter/default-rules/numpy/globals.py      1.00    179.3±3.82µs    16.5 MB/sec    1.04    186.3±7.14µs    15.8 MB/sec
linter/default-rules/pydantic/types.py     1.00      3.1±0.01ms     8.3 MB/sec    1.00      3.1±0.02ms     8.3 MB/sec

Windows

group                                      main                                   pr
-----                                      ----                                   --
formatter/large/dataset.py                 1.02     10.3±0.14ms     3.9 MB/sec    1.00     10.1±0.05ms     4.0 MB/sec
formatter/numpy/ctypeslib.py               1.00  1951.0±32.30µs     8.5 MB/sec    1.00  1946.7±42.63µs     8.6 MB/sec
formatter/numpy/globals.py                 1.00    197.7±2.73µs    14.9 MB/sec    1.00    198.3±4.37µs    14.9 MB/sec
formatter/pydantic/types.py                1.01      4.3±0.06ms     5.9 MB/sec    1.00      4.3±0.02ms     6.0 MB/sec
linter/all-rules/large/dataset.py          1.00     12.7±0.12ms     3.2 MB/sec    1.01     12.8±0.19ms     3.2 MB/sec
linter/all-rules/numpy/ctypeslib.py        1.00      3.6±0.02ms     4.7 MB/sec    1.01      3.6±0.03ms     4.6 MB/sec
linter/all-rules/numpy/globals.py          1.00    370.0±4.61µs     8.0 MB/sec    1.01    372.7±5.27µs     7.9 MB/sec
linter/all-rules/pydantic/types.py         1.00      6.6±0.04ms     3.9 MB/sec    1.01      6.7±0.15ms     3.8 MB/sec
linter/default-rules/large/dataset.py      1.00      6.9±0.04ms     5.9 MB/sec    1.00      6.9±0.05ms     5.9 MB/sec
linter/default-rules/numpy/ctypeslib.py    1.00  1444.5±11.40µs    11.5 MB/sec    1.00  1443.8±12.98µs    11.5 MB/sec
linter/default-rules/numpy/globals.py      1.00    149.8±2.06µs    19.7 MB/sec    1.01    151.1±3.99µs    19.5 MB/sec
linter/default-rules/pydantic/types.py     1.00      3.1±0.02ms     8.2 MB/sec    1.01      3.1±0.05ms     8.1 MB/sec