Enable checking for f strings

[nielsbuwen]

This implements #17714

Support for f-string checking was already partially implemented. Consider this code:

f"{123:abc}"
"{:abc}".format(123)

Mypy ran these lines through the format checker. The call expressions look like this:

# f-string
CallExpr:1(
  MemberExpr:1(
    StrExpr({:{}})
    format)
  Args(
    IntExpr(123)
    StrExpr(abc)))

# format
CallExpr:2(
  MemberExpr:2(
    StrExpr({:abc})
    format)
  Args(
    IntExpr(123)))

The difference is, that the actually static "abc" is converted to a "dynamic" spec.

This MR attempts to enable type checking of f-strings by inlining these "semi dynamic" specs.

Open Questions:

• is this a good idea? Or should the actual parsing of f-strings be changed?
• the test i added always passes on my machine: pytest mypy/test/testcheck.py::TypeCheckSuite::check-string-format.test. It never finds any errors even when it should clearly fail

Checklist

• Read the Contributing Guidelines
• Add tests for all changed behavior
• Make sure CI passes
• Do not force push to PR once reviewed

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

websockets (https://github.com/aaugustin/websockets)
+ src/websockets/sync/messages.py:102: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/tests/scalar/test_na_scalar.py:38: error: Unrecognized format specification "xxx"  [str-format]

pwndbg (https://github.com/pwndbg/pwndbg)
+ pwndbg/gdblib/symbol.py: note: In function "get":
+ pwndbg/gdblib/symbol.py:100: error: Incompatible types in string interpolation (expression has type "Value", placeholder has type "int")  [str-format]

freqtrade (https://github.com/freqtrade/freqtrade)
+ freqtrade/persistence/trade_model.py:852: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]
+ freqtrade/rpc/telegram.py:399: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]
+ freqtrade/rpc/telegram.py:421: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]
+ freqtrade/rpc/telegram.py:446: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]
+ freqtrade/strategy/interface.py:1511: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int | float")  [str-format]

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/connector.py:1164:32: error: Incompatible types in string interpolation (expression has type "None", placeholder has type "int")  [str-format]
+ aiohttp/connector.py:1164:32: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-str-format for more info

materialize (https://github.com/MaterializeInc/materialize)
+ misc/python/materialize/feature_benchmark/comparator.py:56: error: Incompatible types in string interpolation (expression has type "T", placeholder has type "int | float")  [str-format]
+ misc/python/materialize/feature_benchmark/comparator.py:65: error: Incompatible types in string interpolation (expression has type "T", placeholder has type "int | float")  [str-format]

ignite (https://github.com/pytorch/ignite)
+ ignite/utils.py:134: error: Incompatible types in string interpolation (expression has type "Number", placeholder has type "int | float")  [str-format]
+ ignite/utils.py:165: error: Incompatible types in string interpolation (expression has type "Number", placeholder has type "int | float")  [str-format]