Try to install pytest using pip

We need to fix looking for the correct dir
to be added to PYTHONPATH to bootstrap py-test

.github/workflows/unit_tests.yaml

@@ -62,7 +62,7 @@ jobs:
         sudo apt-get install -y coreutils ninja-build graphviz
     - name: Install Python packages
       run: |
-        pip install --upgrade pip six setuptools
+        pip install --upgrade pip pytest six setuptools
         pip install --upgrade -r lib/spack/docs/requirements.txt
     - name: Build documentation
       run: |
@@ -137,7 +137,7 @@ jobs:
           sudo apt-get -y install zlib1g-dev libdw-dev libiberty-dev
     - name: Install Python packages
       run: |
-          pip install --upgrade pip six setuptools codecov coverage[toml]
+          pip install --upgrade pip six setuptools pytest codecov coverage[toml]
           # ensure style checks are not skipped in unit tests for python >= 3.6
           # note that true/false (i.e., 1/0) are opposite in conditions in python and bash
           if python -c 'import sys; sys.exit(not sys.version_info >= (3, 6))'; then
@@ -331,7 +331,7 @@ jobs:
           make -C ${KCOV_ROOT}/build && sudo  make -C ${KCOV_ROOT}/build install
     - name: Install Python packages
       run: |
-          pip install --upgrade pip six setuptools codecov coverage[toml] clingo
+          pip install --upgrade pip pytest six setuptools codecov coverage[toml] clingo
     - name: Setup git configuration
       run: |
           # Need this for the git tests to succeed.
@@ -374,7 +374,7 @@ jobs:
     - name: Install Python packages
       run: |
           pip install --upgrade pip six setuptools
-          pip install --upgrade codecov coverage[toml]
+          pip install --upgrade pytest codecov coverage[toml]
     - name: Setup Homebrew packages
       run: |
         brew install dash fish gcc gnupg2 kcov