Fix format script (#918)

* remove version check for `pipx`

Does not appear needed. `pkg_resources` is removed in Python 3.12

* add `MONGOCRYPT_PYTHON` to use a specified python install

* add a CONTRIBUTING.md

Author: kevinAlbs

.evergreen/init.sh

@@ -222,6 +222,7 @@ run_ctest() {
 
 run_python() {
     pys=(
+        "${MONGOCRYPT_PYTHON:-}"
         py
         python3.14
         python3.13

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# Formatting
+
+To format, install `pipx` and run:
+```bash
+./etc/format-all.sh
+```
+
+To use a specified python install, set `MONGOCRYPT_PYTHON`:
+
+```bash
+# Set MONGOCRYPT_PYTHON to a python install with pipx installed.
+export MONGOCRYPT_PYTHON=python
+./etc/format-all.sh
+```

etc/format.sh

@@ -9,8 +9,6 @@ if ! run_python -c ''; then
   fail "No Python found?"
 fi
 
-# Check that we have a pipx of the proper version:
-run_python -c 'import pkg_resources; pkg_resources.require("pipx>=0.17.0,<2.0")'
 
 # Give default clang-format an empty string on stdin if there are no inputs files
 printf '' | run_python -m pipx run "clang-format==${CLANG_FORMAT_VERSION:?}" "$@"