Remove getdeps dependency on Python 3.8

Summary:
X-link: facebookincubator/zstrong#1175

Instead of having fbthrift download and build Python 3.8 (which is ancient), users should be able to build fbthrift against the target Python environment. (This is consistent with how Python packages would normally be built/installed.)

Note that users can specify their target version of Python via the `PYTHON_LIBRARY` and `PYTHON_INCLUDE_DIR` CMake variables, as demonstrated in the command in the test plan.

Reviewed By: vladmihailescu

Differential Revision: D68510340

fbshipit-source-id: f94805de4220b2c107ed0aa0d57fd23b65b29bd2

build/fbcode_builder/manifests/fbthrift

@@ -31,11 +31,6 @@ xxhash
 # specific configuration we rely on the folly manifest to provide this
 # dependency to avoid duplication.
 
-[dependencies.os=linux]
-# python doesn't build on Windows yet and this causes python3 shebangs to
-# expand to a non-portable path on macOS
-python
-
 [shipit.pathmap]
 xplat/thrift/public_tld = .
 xplat/thrift = thrift