Set MPLBACKEND=Agg in CI to fix Tkinter issues on Windows

On Windows Python 3.13 CI environment, Tcl/Tk is not properly configured,
causing matplotlib tests to fail with TclError. Setting the MPLBACKEND
environment variable to 'Agg' (non-interactive backend) resolves this issue.

Changes:
- Added MPLBACKEND=Agg env var to all test jobs (Linux, macOS, Windows)
- This ensures consistent behavior across all CI platforms
- Fixes TclError in test_phase_plane.py and test_aligns.py on Windows

This is a cleaner solution than modifying individual test files.

Author: Routhleck

.github/workflows/CI.yml

@@ -48,6 +48,8 @@ jobs:
           if [ -f requirements-dev.txt ]; then pip install -r requirements-dev.txt; fi
           pip install -e .
       - name: Test with pytest
+        env:
+          MPLBACKEND: Agg  # Use non-interactive backend for matplotlib
         run: |
           pytest brainpy/
 
@@ -77,6 +79,8 @@ jobs:
           if [ -f requirements-dev.txt ]; then pip install -r requirements-dev.txt; fi
           pip install -e .
       - name: Test with pytest
+        env:
+          MPLBACKEND: Agg  # Use non-interactive backend for matplotlib
         run: |
           pytest brainpy/
 
@@ -106,5 +110,7 @@ jobs:
           python -m pip install -r requirements-dev.txt
           pip install -e .
       - name: Test with pytest
+        env:
+          MPLBACKEND: Agg  # Use non-interactive backend for matplotlib
         run: |
           pytest brainpy/