fbcode_builder: drop Python 2 references from getdeps.py comments

Summary:
We dropped Python 2 support a long time ago. Two comment blocks in the
GHA workflow generator still talk about Python 2:

- The "python may be version 2 or version 3 depending on the system"
  block predates dropping Py2 support and the warning about `python3
  may not be a thing` is no longer relevant on any supported runner.
- The Windows comment claims `python2.exe is available if needed`,
  which is false and misleading — the GHA Windows runners ship Python
  3 only and expose it as `python`.

Replace both with comments that describe what the code actually does
today: pick a per-OS interpreter name (`python3` on Linux/macOS,
`python` on Windows) for the generated workflows. The behavior is
unchanged.

Reviewed By: sahilsd

Differential Revision: D101585861

fbshipit-source-id: db7c9aae842efdf920c84b297f2f6148cc0e1197

Author: r-barnes

build/fbcode_builder/getdeps.py

@@ -1112,11 +1112,9 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
         if builder_name == "nop":
             return None
 
-        # We want to be sure that we're running things with python 3
-        # but python versioning is honestly a bit of a frustrating mess.
-        # `python` may be version 2 or version 3 depending on the system.
-        # python3 may not be a thing at all!
-        # Assume an optimistic default
+        # The interpreter name varies across runners, so resolve it per-OS
+        # below. Default to `python3`, which works on Linux and macOS GHA
+        # runners.
         py3 = "python3"
 
         if build_opts.is_linux():
@@ -1133,8 +1131,8 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
                 runs_on = args.runs_on
             else:
                 runs_on = "windows-2022"
-            # The windows runners are python 3 by default; python2.exe
-            # is available if needed.
+            # On Windows GHA runners the python 3 interpreter is exposed as
+            # `python`, not `python3`.
             py3 = "python"
         else:
             artifacts = "mac"