[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/codegen/python.h

@@ -0,0 +1,45 @@
+#ifndef FLATBUFFERS_INCLUDE_CODEGEN_PYTHON_H_
+#define FLATBUFFERS_INCLUDE_CODEGEN_PYTHON_H_
+
+#include <cstdint>
+#include <set>
+#include <string>
+#include <vector>
+
+namespace flatbuffers {
+namespace python {
+// `Version` represent a Python version.
+//
+// The zero value (i.e. `Version{}`) represents both Python2 and Python3.
+//
+// https://docs.python.org/3/faq/general.html#how-does-the-python-version-numbering-scheme-work
+struct Version {
+  explicit Version(const std::string &version);
+
+  bool IsValid() const;
+
+  int16_t major = 0;
+  int16_t minor = 0;
+  int16_t micro = 0;
+};
+
+std::set<std::string> Keywords(const Version &version);
+
+struct Import {
+  bool IsLocal() const { return module == "."; }
+
+  std::string module;
+  std::string name;
+};
+
+struct Imports {
+  const python::Import &Import(const std::string &module);
+  const python::Import &Import(const std::string &module,
+                               const std::string &name);
+
+  std::vector<python::Import> imports;
+};
+}  // namespace python
+}  // namespace flatbuffers
+
+#endif  // FLATBUFFERS_INCLUDE_CODEGEN_PYTHON_H_