ci: use uv in place of pip to speed up installing dependencies

uv cuts down time needed to install dependencies from ~2m to ~37s.
Moreover, once the runner/environment is cached, it's like instant.

Drops python3.6 from CI, uv pip doesn't seem support python3.6 as CI
job fails.

.github/workflows/ci.yml

@@ -5,6 +5,10 @@ defaults:
   run:
     shell: bash
 
+env:
+  # https://docs.astral.sh/uv/guides/integration/github/#using-uv-pip
+  UV_SYSTEM_PYTHON: 1
+
 jobs:
   unit-tests:
     name: "Unit Tests"
@@ -14,8 +18,6 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-20.04
-            python-version: "3.6"
           - os: ubuntu-20.04
             python-version: "3.7"
           - os: ubuntu-latest
@@ -37,20 +39,17 @@ jobs:
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v2
-        with:
-          python-version: ${{ matrix.python-version }}
 
-      - name: Upgrade pip
-        run: |
-          python -m pip install -U pip
+      - name: Install uv
+        uses: astral-sh/setup-uv@v5
 
       - name: Configure environments
         run: |
           python --version
 
       - name: Install dependencies
         run: |
-          pip install -e ".[test]"
+          uv pip install -e ".[test]"
           python -m imgcat --version
 
       - name: Run tests

setup.py

@@ -28,9 +28,9 @@ def read_version():
 
 tests_requires = [
     'pytest',
-    'numpy',
-    'torch',
-    'tensorflow>=2.0',
+    'numpy>=1.21,<2.0',
+    'torch>=2.0,<3.0; python_version >= "3.8"',
+    'tensorflow>=2.0,<3.0; python_version >= "3.8"',
     'matplotlib>=3.3',
     'Pillow',
 ]