fix: remove libcst as a required dependency (#389)

* Remove libcst dependency

libcst is only used for a one-off "fixup" script. It’s not necessary to always depend on it.

It caused an issue on one project as old libcst versions don't ship macOS M1 wheels, and it took time to figure this out.

(Also the instructions for using the fixup script were incorrect.)

* Update UPGRADING.md

Co-authored-by: Tim Swast <[email protected]>

Author: adamchainz

UPGRADING.md

@@ -132,11 +132,12 @@ wrapper around the auto-generated `read_rows()` method.
 python3 -m pip install google-cloud-bigquery-storage
 ```
 
-* The script `fixup_storage_v1_keywords.py` is shipped with the library. It expects
-an input directory (with the code to convert) and an empty destination directory.
+* The script `fixup_bigquery_storage_v1_keywords.py` is shipped with the library. It
+requires `libcst` to be installed. It expects an input directory (with the code
+to convert) and an empty destination directory.
 
 ```sh
-$ scripts/fixup_storage_v1_keywords.py --input-directory .samples/ --output-directory samples/
+$ fixup_bigquery_storage_v1_keywords.py --input-directory .samples/ --output-directory samples/
 ```
 
 **Before:**

setup.py

@@ -29,7 +29,6 @@
     # https://github.com/googleapis/google-cloud-python/issues/10566
     "google-api-core[grpc] >= 1.28.0, <3.0.0dev",
     "proto-plus >= 1.18.0",
-    "libcst >= 0.2.5",
 ]
 extras = {
     "pandas": ["pandas>=0.21.1"],