update python to 3.8

Summary:
Update to a newer python that builds on macOS Big Sur, make eden_scm depend on it and use it from PATH

python 3.8 requires libffi which is reference via its pkg-config setup,  however python's pkg-config libffi detection is broken (https://bugs.python.org/issue34823) with the documented workaround requiring an environment variable to be passed into its ./configure step, which is why this change also adds a feature to AutoconfBuilder

With the updated python in place I was able to remove disable_env_override_pkgconfig = 1
disable_env_override_path = 1 from the eden_scm config so that it actually uses the pkg-configs, PATHs and other settings from its dependencies. This should make future python and other dependency upgrades much simpler.

Reviewed By: HarveyHunt

Differential Revision: D32231261

fbshipit-source-id: a2b6addbe22f38e3d71618c802d2c6f836fdd86c

build/fbcode_builder/manifests/python

@@ -8,10 +8,27 @@ python3-devel
 [debs]
 python3-all-dev
 
-[download.os=linux]
-url = https://www.python.org/ftp/python/3.7.6/Python-3.7.6.tgz
-sha256 = aeee681c235ad336af116f08ab6563361a0c81c537072c1b309d6e4050aa2114
+[download]
+url = https://www.python.org/ftp/python/3.8.12/Python-3.8.12.tgz
+sha256 = 316aa33f3b7707d041e73f246efedb297a70898c4b91f127f66dc8d80c596f1a
 
-[build.os=linux]
+[build]
 builder = autoconf
-subdir = Python-3.7.6
+subdir = Python-3.8.12
+
+[autoconf.args]
+--enable-shared
+--with-ensurepip=install
+
+# python's pkg-config libffi detection is broken
+# See https://bugs.python.org/issue34823 for clearest description
+# and pending PR https://github.com/python/cpython/pull/20451
+# The documented workaround requires an environment variable derived from
+# pkg-config to be passed into its configure step
+[autoconf.envcmd.LDFLAGS]
+pkg-config
+--libs-only-L
+libffi
+
+[dependencies]
+libffi