[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.cc

@@ -0,0 +1,63 @@
+#include "codegen/python.h"
+
+#include <set>
+#include <sstream>
+#include <string>
+#include <utility>
+
+namespace flatbuffers {
+namespace python {
+Version::Version(const std::string &version) {
+  std::stringstream ss(version);
+  char dot;
+  ss >> major >> dot >> minor >> dot >> micro;
+}
+
+bool Version::IsValid() const {
+  return (major == 0 || major == 2 || major == 3) && minor >= 0 && micro >= 0;
+}
+
+std::set<std::string> Keywords(const Version &version) {
+  switch (version.major) {
+    case 2:
+      // https://docs.python.org/2/reference/lexical_analysis.html#keywords
+      return {
+          "and",   "as",     "assert", "break",  "class", "continue", "def",
+          "del",   "elif",   "else",   "except", "exec",  "finally",  "for",
+          "from",  "global", "if",     "import", "in",    "is",       "lambda",
+          "not",   "or",     "pass",   "print",  "raise", "return",   "try",
+          "while", "with",   "yield",
+      };
+    case 0:
+    case 3:
+      // https://docs.python.org/3/reference/lexical_analysis.html#keywords
+      return {
+          "and",      "as",       "assert",  "async", "await",  "break",
+          "class",    "continue", "def",     "del",   "elif",   "else",
+          "except",   "False",    "finally", "for",   "from",   "global",
+          "if",       "import",   "in",      "is",    "lambda", "None",
+          "nonlocal", "not",      "or",      "pass",  "raise",  "return",
+          "True",     "try",      "while",   "with",  "yield",
+      };
+    default:
+      return {};
+  }
+}
+
+const python::Import &python::Imports::Import(const std::string &module) {
+  python::Import import;
+  import.module = module;
+  imports.push_back(std::move(import));
+  return imports.back();
+}
+
+const python::Import &python::Imports::Import(const std::string &module,
+                                              const std::string &name) {
+  python::Import import;
+  import.module = module;
+  import.name = name;
+  imports.push_back(std::move(import));
+  return imports.back();
+}
+}  // namespace python
+}  // namespace flatbuffers