allow macOS installer builds to package pre-built html docs (GH-20715)

build-installer now looks in its directory of source tarballs
for a suitable html tarball of the same version.  If so, it
will unpack and use it rather than rebuilding the html format
documentation set from the source repo.  This is intended as
a speedup for test builds of the installer.  Files names must
be in the same format as produced by the docs build for download,
for example, `python-3.9.0b1-docs-html.tar.bz2`.

Mac/BuildScript/build-installer.py

@@ -1066,14 +1066,40 @@ def buildPythonDocs():
     curDir = os.getcwd()
     os.chdir(buildDir)
     runCommand('make clean')
+
+    # Search third-party source directory for a pre-built version of the docs.
+    #   Use the naming convention of the docs.python.org html downloads:
+    #       python-3.9.0b1-docs-html.tar.bz2
+    doctarfiles = [ f for f in os.listdir(DEPSRC)
+        if f.startswith('python-'+getFullVersion())
+        if f.endswith('-docs-html.tar.bz2') ]
+    if doctarfiles:
+        doctarfile = doctarfiles[0]
+        if not os.path.exists('build'):
+            os.mkdir('build')
+        # if build directory existed, it was emptied by make clean, above
+        os.chdir('build')
+        # Extract the first archive found for this version into build
+        runCommand('tar xjf %s'%shellQuote(os.path.join(DEPSRC, doctarfile)))
+        # see if tar extracted a directory ending in -docs-html
+        archivefiles = [ f for f in os.listdir('.')
+            if f.endswith('-docs-html')
+            if os.path.isdir(f) ]
+        if archivefiles:
+            archivefile = archivefiles[0]
+            # make it our 'Docs/build/html' directory
+            print(' -- using pre-built python documentation from %s'%archivefile)
+            os.rename(archivefile, 'html')
+        os.chdir(buildDir)
+
+    htmlDir = os.path.join('build', 'html')
+    if not os.path.exists(htmlDir):
         # Create virtual environment for docs builds with blurb and sphinx
         runCommand('make venv')
         runCommand('venv/bin/python3 -m pip install -U Sphinx==2.2.0')
         runCommand('make html PYTHON=venv/bin/python')
+    os.rename(htmlDir, docdir)
     os.chdir(curDir)
-    if not os.path.exists(docdir):
-        os.mkdir(docdir)
-    os.rename(os.path.join(buildDir, 'build', 'html'), docdir)
 
 
 def buildPython():