chore: move libcst requirement to extras (#50)

The package uses libcst only for the optional 1.0 -> 2.0 fixer script, so it should not be a hard runtime requirement.

This avoids apps using the package pulling in `pyyaml`, `typing-inspect` and `typing-extensions` too, which is a big win in my books.

UPGRADING.md

@@ -20,7 +20,7 @@ Methods expect request objects. We provide a script that will convert most commo
 * Install the library
 
 ```py
-python3 -m pip install google-cloud-iam
+python3 -m pip install google-cloud-iam[fixup]
 ```
 
 * The script `fixup_credentials_v1_keywords.py` is shipped with the library. It expects an input directory (with the code to convert) and an empty destination directory.

docs/UPGRADING.md

@@ -20,7 +20,7 @@ Methods expect request objects. We provide a script that will convert most commo
 * Install the library
 
 ```py
-python3 -m pip install google-cloud-iam
+python3 -m pip install google-cloud-iam[fixup]
 ```
 
 * The script `fixup_credentials_v1_keywords.py` is shipped with the library. It expects an input directory (with the code to convert) and an empty destination directory.

scripts/fixup_credentials_v1_keywords.py

@@ -18,11 +18,16 @@
 
 import argparse
 import os
-import libcst as cst
 import pathlib
 import sys
 from typing import (Any, Callable, Dict, List, Sequence, Tuple)
 
+try:
+    import libcst as cst
+except ImportError:
+    print("*** Could not import libcst! Did you install the google-cloud-iam package with the `[fixup]` extra?")
+    raise
+
 
 def partition(
     predicate: Callable[[Any], bool],

setup.py

@@ -30,7 +30,6 @@
 dependencies = [
     "google-api-core[grpc] >= 1.22.0, < 2.0.0dev",
     "proto-plus >= 0.4.0",
-    "libcst >= 0.2.5",
 ]
 
 package_root = os.path.abspath(os.path.dirname(__file__))
@@ -74,6 +73,7 @@
     packages=packages,
     namespace_packages=namespaces,
     install_requires=dependencies,
+    extras_require={"fixup": ["libcst >= 0.2.5"]},
     python_requires=">=3.6",
     scripts=["scripts/fixup_credentials_v1_keywords.py"],
     include_package_data=True,