Fix errors arising under formulaic>=1.1.0

[s3alfisc]

For details, see here and here.

Reason for the bug: formulaic no longer adds a T. prefix before categorical variables. As a result, regex operations failed.
This led to loud errors in the fixef() methods, but silent failures when variables where interacted via i() with a reference level set via i(..., ref): the reference level would not be dropped from the design matrix X.

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Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyfixest/utils/dev_utils.py	85.71%	1 Missing ⚠️

Flag	Coverage Δ
core-tests	82.55% <87.50%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
pyfixest/estimation/model_matrix_fixest_.py	85.32% <100.00%> (ø)
pyfixest/utils/dev_utils.py	83.78% <85.71%> (-0.43%)	⬇️