bpo-31036: Allow sphinx and blurb to be found automatically (#3440)
Rather than requiring the path to blurb and/or sphinx-build to be specified to the make rule, enhance the Doc/Makefile to look for each first in a virtual environment created by make venv and, if not found, look on the normal process PATH. This allows the Doc/Makefile to take advantage of an installed spinx-build or blurb and, thus, do the right thing most of the time. Also, make the directory for the venv be configurable and document the `make venv` target.
diff --git a/.gitignore b/.gitignore
index ea7abfe..5920654 100644
--- a/.gitignore
+++ b/.gitignore
@@ -19,6 +19,9 @@
.gdb_history
Doc/build/
Doc/venv/
+Doc/.venv/
+Doc/env/
+Doc/.env/
Include/pydtrace_probes.h
Lib/distutils/command/*.pdb
Lib/lib2to3/*.pickle