bpo-42504: fix for MACOSX_DEPLOYMENT_TARGET=11 (GH-23556)

bpo-42504: fix for MACOSX_DEPLOYMENT_TARGET=11 (GH-23556)

macOS releases numbering has changed as of macOS 11 Big Sur.  Previously, major releases were of the form 10.x, 10.x+1, 10.x+2, etc; as of Big Sur, they are now x, x+1, etc, so, for example, 10.15, 10.15.1, ..., 10.15.7, 11, 11.0.1, 11.1, ..., 12, 12.1, etc. Allow Python to build with single-digit deployment target values. Patch provided by FX Coudert.
(cherry picked from commit 5291639e611dc3f55a34666036f2c3424648ba50)

Co-authored-by: FX Coudert <fxcoudert@gmail.com>