To facilitate cleaner updates of the externally-maintained libmpdec required by the _decimal module, we should migrate away from the bundled copy in Modules/_decimal/libmpdec and towards an "external" in cpython-source-deps for Windows and --with-system-libmpdec by default elsewhere.

My tentative plan is as follows:

• In 3.13:
  • Add mpdecimal-2.5.1 and mpdecimal-4.0.0 to cpython-source-deps:
    • Sort list of downloads, and add link to mpdecimal sources cpython-source-deps#32
    • https://github.com/python/cpython-source-deps/tree/mpdecimal
  • Switch Windows build to mpdecimal-2.5.1 external (gh-115119: Switch Windows build to mpdecimal external #115182)
  • gh-115119: Detect _decimal dependencies using pkg-config #115406
  • Update deps and build instructions wrt. libmpdecimal devguide#1313
  • gh-115119: Update macOS installer to build with libmpdec 4.0.0 #118382
  • gh-115119: Update Windows installer to mpdecimal 4.0.0 #118506
  • Update macOS build to mpdecimal-4.0.0
  • gh-115119: Default to --with-system-libmpdec=yes #118539
  • gh-115119: Bump CI to use Ubuntu 22.04 #118631
  • Post #118539: amend Ubuntu CI to use system installed mpdecimal
  • Update the docs; see Unvendor libmpdec sources #115119 (comment)
    • gh-115119: recommend upstream libmpdec in build requirements #136205
    • Update libmpdec-related instructions devguide#1598
  • gh-115119: Fall back to bundled libmpdec if system libmpdec is not found #119196
• In 3.15:
  • Defer removal of bundled libmpdec in 3.15: gh-115119: defer removal of bundled libmpdec to 3.16 #133997
  • Remove the implicit fallback to the bundled copy, keep support for explicit --without-system-libmpdec (with warning): gh-115119: removed implicit fallback to the bundled libmpdec #134078
• In 3.16:
  • Remove --with[out]-system-libmpdec configure options and Modules/_decimal/libmpdec/: gh-115119: Removed bundled copy of the libmpdec #133964

Linked PRs

• gh-115119: Switch Windows build to mpdecimal external #115182
• gh-115119: Detect _decimal dependencies using pkg-config #115406
• gh-115119: Update macOS installer to build with libmpdec 4.0.0 #118382
• gh-115119: Update Windows installer to mpdecimal 4.0.0 #118506
• gh-115119: Default to --with-system-libmpdec=yes #118539
• gh-115119: Bump CI to use Ubuntu 22.04 #118631
• [3.12] gh-115119: Bump CI to use Ubuntu 22.04 (#118631) #118634
• gh-115119: Fall back to bundled libmpdec if system libmpdec is not found #119196
• [3.13] gh-115119: Fall back to bundled libmpdec if system libmpdec is not found (GH-119196) #119217
• gh-115119: Removed bundled copy of the libmpdec #133964
• gh-115119: defer removal of bundled libmpdec to 3.16 #133997
• gh-115119: removed implicit fallback to the bundled libmpdec #134078
• gh-115119: removed implicit fallback to the bundled libmpdec #135568
• [3.14] gh-115119: Defer removal of bundled libmpdec to 3.16 (GH-133997) #136199
• [3.13] gh-115119: Defer removal of bundled libmpdec to 3.16 (GH-133997) #136201
• gh-115119: recommend upstream libmpdec in build requirements #136205
• gh-115119: Document libmpdec dependency in the build doc #136239
• [3.14] gh-115119: Recommend upstream libmpdec in build requirements (GH-136205) #136244
• [3.13] gh-115119: Recommend upstream libmpdec in build requirements (#136205) #136245