ci: use installed pip, upgrade to pinned if necessary

Ref: pytest-dev#6522

Author: blueyed

.github/workflows/main.yml

@@ -122,7 +122,7 @@ jobs:
         python-version: ${{ matrix.python }}
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
+        python -m pip --version
         pip install tox coverage
 
     - name: Test without coverage
@@ -173,7 +173,7 @@ jobs:
         python-version: "3.7"
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
+        python -m pip --version
         pip install --upgrade wheel setuptools tox
     - name: Build package
       run: |

.travis.yml

@@ -43,7 +43,7 @@ jobs:
       python: '3.5.1'
       dist: trusty
       before_install:
-        - python -m pip install -U pip
+        - python -m pip install -U pip==19.3.1
 
     - env: TOXENV=linting,docs,doctesting
       cache: