[Python] Generate .pyi stub files when --python-typing is on. (#8312)

* [Python] Generate `.pyi` stub files when `--python-typing` is on.

To support this change, the following modifications were made:

-  added a new option to disable `numpy` helpers generation;
-  added a new flag to control the target Python version:

   `--python-version` can be one of the following:

   - `0.x.x` – compatible with any Python version;
   - `2.x.x` – compatible with Python 2;
   - `3.x.x` – compatible with Python 3.
-  added codegen utilities for Python;
-  added a note that the generated .py file is empty.

* [Python] Update Bazel build rules.

* [Python] Update Bazel build rules.

* [Python] Run buildifier on BUILD.bazel files.

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

include/flatbuffers/idl.h

@@ -706,8 +706,26 @@ struct IDLOptions {
   bool no_leak_private_annotations;
   bool require_json_eof;
   bool keep_proto_id;
+
+  /********************************** Python **********************************/
   bool python_no_type_prefix_suffix;
   bool python_typing;
+
+  // The target Python version. Can be one of the following:
+  // -  "0"
+  // -  "2"
+  // -  "3"
+  // -  "2.<minor>"
+  // -  "3.<minor>"
+  // -  "2.<minor>.<micro>"
+  // -  "3.<minor>.<micro>"
+  //
+  // https://docs.python.org/3/faq/general.html#how-does-the-python-version-numbering-scheme-work
+  std::string python_version;
+
+  // Whether to generate numpy helpers.
+  bool python_gen_numpy;
+
   bool ts_omit_entrypoint;
   ProtoIdGapAction proto_id_gap_action;
 
@@ -828,6 +846,7 @@ struct IDLOptions {
         keep_proto_id(false),
         python_no_type_prefix_suffix(false),
         python_typing(false),
+        python_gen_numpy(true),
         ts_omit_entrypoint(false),
         proto_id_gap_action(ProtoIdGapAction::WARNING),
         mini_reflect(IDLOptions::kNone),