Bug 1714641: Remove usages of vendored "backports" code r=ahal,perftest-reviewers

It provides `shutil.which(...)`, which has been part of `shutil` since
Python 3.3.

Differential Revision: https://phabricator.services.mozilla.com/D117063

build/moz.configure/init.configure

@@ -291,7 +291,6 @@ def virtualenv_python3(env_python, build_env, mozconfig, help):
 
     # Update the path to include some necessary modules for find_program.
     sys.path.insert(0, os.path.join(topsrcdir, "testing", "mozbase", "mozfile"))
-    sys.path.insert(0, os.path.join(topsrcdir, "third_party", "python", "backports"))
 
     # If we know the Python executable the caller is asking for then verify its
     # version. If the caller did not ask for a specific executable then find

docs/code-quality/lint/create.rst

@@ -111,14 +111,10 @@ let's call the file ``flake8_lint.py`` (`in-tree version <https://searchfox.org/
     import os
     import subprocess
     from collections import defaultdict
+    from shutil import which
 
     from mozlint import result
 
-    try:
-        from shutil import which
-    except ImportError:
-        from shutil_which import which
-
 
     FLAKE8_NOT_FOUND = """
     Could not find flake8! Install flake8 and try again.

testing/mozbase/mozfile/mozfile/mozfile.py

@@ -421,10 +421,7 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None, exts=None, extra_search_dirs=(
     extra_search_dirs is a convenience argument. If provided, the strings in
     the sequence will be appended to the END of the given `path`.
     """
-    try:
-        from shutil import which as shutil_which
-    except ImportError:
-        from shutil_which import which as shutil_which
+    from shutil import which as shutil_which
 
     if isinstance(path, (list, tuple)):
         path = os.pathsep.join(path)

tools/browsertime/mach_commands.py

@@ -174,10 +174,7 @@ class MachBrowsertime(MachCommandBase):
         if not AUTOMATION and host_platform().startswith("linux"):
             # On Linux ImageMagick needs to be installed manually, and `mach bootstrap` doesn't
             # do that (yet).  Provide some guidance.
-            try:
-                from shutil import which
-            except ImportError:
-                from shutil_which import which
+            from shutil import which
 
             im_programs = ("compare", "convert", "mogrify")
             for im_program in im_programs: