Avoiding pip==20.0 and pip==20.0.1. (#192)

See some recent issues filed with these releases, e.g.
pypa/pip#7629

.appveyor.yml

@@ -21,6 +21,7 @@ environment:
     MINGW_32: C:\mingw-w64\i686-6.3.0-posix-dwarf-rt_v5-rev1\mingw32\bin
     MINGW_64: C:\mingw-w64\x86_64-6.3.0-posix-seh-rt_v5-rev1\mingw64\bin
     DEFAULT_PYTHON: "C:\\Python38-x64"
+    VIRTUALENV_NO_DOWNLOAD: "true"
 
   matrix:
 
@@ -61,7 +62,7 @@ install:
       $env:Path += ";$MINGW"
 
   # Packaging requirements
-  - python -m pip install --upgrade pip setuptools
+  - python -m pip install --upgrade "pip !=20.0,!=20.0.1" setuptools
   - python -m pip install --upgrade wheel
 
   # Install the build dependencies of the project.

.circleci/config.yml

@@ -7,6 +7,7 @@ jobs:
       - image: dhermes/bezier:latest
     environment:
       MATPLOTLIBRC: tests
+      VIRTUALENV_NO_DOWNLOAD: "true"
     steps:
       - checkout
       - setup_remote_docker

.travis.yml

@@ -18,14 +18,17 @@ matrix:
       language: generic
       env:
         - PY_VERSION=3.6
+        - VIRTUALENV_NO_DOWNLOAD=true
     - os: osx
       language: generic
       env:
         - PY_VERSION=3.7
+        - VIRTUALENV_NO_DOWNLOAD=true
     - os: osx
       language: generic
       env:
         - PY_VERSION=3.8
+        - VIRTUALENV_NO_DOWNLOAD=true
 
 install:
   - source scripts/macos/travis-setup.sh

scripts/macos/travis-setup.sh

@@ -39,7 +39,7 @@ export PIP_CMD=${PIP_CMD/sudo/sudo -H}
 echo "PIP_CMD=${PIP_CMD}"
 
 # Make sure our installed CPython is set up for testing.
-${PIP_CMD} install --ignore-installed virtualenv pip
+${PIP_CMD} install --ignore-installed virtualenv "pip !=20.0,!=20.0.1"
 ${PIP_CMD} install --upgrade "nox >= 2019.11.9" numpy
 # Make sure there is a working ``cmake`` on the ``${PATH}``.
 ${PIP_CMD} install --upgrade "cmake >= 3.15.3"