fix: add pip to environment.yml to prevent system pip fallback

[drbenvincent]

Summary

This PR fixes an issue where conda env create -f environment.yml would create an environment that uses the wrong pip executable, causing confusing installation errors.

Problem

When setting up the development environment, users encountered this error:

ERROR: Package 'causalpy' requires a different Python: 3.10.16 not in '>=3.11'

This occurred even when the conda environment had Python 3.12+ installed. The root cause:

1. The environment.yml did not include pip as a dependency
2. Without pip in the conda environment, the shell falls back to the system pip (e.g., Homebrew's /opt/homebrew/bin/pip on macOS)
3. The system pip is linked to a different Python version (3.10), causing the version mismatch error

Solution

1. Added pip as an explicit dependency - This ensures pip is installed within the conda environment and the correct pip is used
2. Bumped minimum Python from 3.11 to 3.12 - Aligns with current development practices and ensures consistent solver behavior

Testing

After this fix:

• which pip correctly shows /path/to/mambaforge/envs/CausalPy/bin/pip
• pip install -e ".[test]" works without version mismatch errors
• make test runs successfully

---

📚 Documentation preview 📚: https://causalpy--632.org.readthedocs.build/en/632/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.74%. Comparing base (f5366c0) to head (7eea1c5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #632      +/-   ##
==========================================
+ Coverage   93.48%   93.74%   +0.26%     
==========================================
  Files          39       41       +2     
  Lines        6031     6827     +796     
  Branches      389      458      +69     
==========================================
+ Hits         5638     6400     +762     
- Misses        255      267      +12     
- Partials      138      160      +22     

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