ci: attempt to use pip cache to speedup

https://github.blog/changelog/2021-11-23-github-actions-setup-python-now-supports-dependency-caching/

UPD: hmm it doesn't seem to work, complains that requirements.txt is missing

this might be relevant...
https://github.com/actions/setup-python/issues/361

.github/workflows/main.yml

@@ -30,6 +30,7 @@ jobs:
     - uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
+        cache: 'pip'
 
     - uses: actions/checkout@v2
       with:
@@ -61,6 +62,7 @@ jobs:
     - uses: actions/setup-python@v2
       with:
         python-version: '3.7'
+        cache: 'pip'
 
     - uses: actions/checkout@v2
       with: