feat: replace pip with uv (#147)

uv is a drop-in replacement for pip for the needs of this action. I got a 3x improvement in performance in my local tests... (from ~30s execution down to ~10s execution) .

`astral-sh/setup-uv`'s cache is disabled because it cannot generate a cache key. 

It cannot because this action does not pin the version of `mergify-cli` (by design I gather), that causes a bunch of warnings and would (potentially) cache a mergify cli version forever, besides uv is so fast and `mergify-cli` is so small there is no benefit for the cache anyway.

Author: samuelhnrq

action.yml

@@ -65,10 +65,17 @@ runs:
       with:
         python-version: "3.14"
 
+    - name: Install uv
+      uses: astral-sh/setup-uv@v7
+      with:
+        # mergify is too small to benefit + there is no version lock (so no cache key either)
+        enable-cache: false
+
     - name: Install dependencies
       shell: bash
       run: |
-        pip install mergify-cli
+        uv tool install mergify-cli
+        mergify --version
 
     - shell: bash
       id: junit-process