Migrate to python3 (#5549) (#5743)

Main swift project moved to python3, moving swiftpm build tools to
python3 to match.

(cherry picked from commit c0d5daa)

Co-authored-by: Evan Wilde <etceterawilde@gmail.com>

Author: shahmishal

Utilities/Docker/Dockerfile

@@ -40,6 +40,6 @@ RUN wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/nul
 RUN apt-add-repository "deb https://apt.kitware.com/ubuntu/ $ubuntu_version main"
 
 RUN apt-get install -y \
-  python \
+  python3 \
   cmake \
   ninja-build

Utilities/bootstrap

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # encoding: utf-8
 
 """

Utilities/helpers.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 ##===----------------------------------------------------------------------===##
 ##
 ## This source file is part of the Swift open source project

Utilities/soundness.sh

@@ -86,7 +86,7 @@ EOF
         exceptions=( -path "./Examples/*" -o -path "./Fixtures/*" -o -path "./IntegrationTests/*" )
         matching_files=( -name '*.py' )
         cat > "$tmp" <<"EOF"
-#!/usr/bin/env python
+#!/usr/bin/env python3
 ##===----------------------------------------------------------------------===##
 ##
 ## This source file is part of the Swift open source project

Utilities/test-toolchain

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 ##===----------------------------------------------------------------------===##
 ##
 ## This source file is part of the Swift open source project