Always use latest version of `pip`. (#2396)

Installation should start by updating `pip` to the latest version.

Resolves issue noted in https://github.com/encode/httpx/pull/2334#issuecomment-1268308195

diff --git a/scripts/install b/scripts/install
index 65885a7208961207ed34c9659c21ac5ef0b90314..f98bbfca3c844877a83d220042a2abcd033176a3 100755 (executable)
--- a/scripts/install
+++ b/scripts/install
@@ -15,5 +15,6 @@ else
     PIP="pip"
 fi
 
+"$PIP" install -U pip
 "$PIP" install -r "$REQUIREMENTS"
 "$PIP" install -e .