heartbeat: migrate to python@3.9

[fxcoudert]

As part of the Python 3.9 migration (#62201).

This formula is independent from the all other Python formulas (if I didn't screw up my script or my logic)

Do not merge before the next Brew tag ships, expected on Monday 2020-10-12

[fxcoudert]

    The headers or library files could not be found for zlib,
    a required dependency when compiling Pillow from source.

[yous]

Similar to #62243 (comment):

diff --git a/Formula/heartbeat.rb b/Formula/heartbeat.rb
index bd813a0249..19ef579613 100644
--- a/Formula/heartbeat.rb
+++ b/Formula/heartbeat.rb
@@ -16,7 +16,9 @@ class Heartbeat < Formula
   end
 
   depends_on "go" => :build
+  depends_on "jpeg" => :build
   depends_on "python@3.9" => :build
+  depends_on "zlib" => :build
 
   resource "virtualenv" do
     url "https://files.pythonhosted.org/packages/b1/72/2d70c5a1de409ceb3a27ff2ec007ecdd5cc52239e7c74990e32af57affe9/virtualenv-15.2.0.tar.gz"
@@ -50,7 +52,7 @@ class Heartbeat < Formula
     cd "src/github.com/elastic/beats/heartbeat" do
       system "make", "mage"
       # prevent downloading binary wheels during python setup
-      system "make", "PIP_INSTALL_PARAMS=--no-binary :all", "python-env"
+      system "make", "PIP_INSTALL_PARAMS=--use-pep517 --no-binary :all:", "python-env"
       system "mage", "-v", "build"
       ENV.deparallelize
       system "mage", "-v", "update"

[fxcoudert]

• There is a system zlib, which it should find
• Why do we need jpeg at build-time only? That's unusual

[yous]

• make python-env seems to install Pillow, and Pillow tries to find libjpeg, zlib, etc. https://pillow.readthedocs.io/en/latest/installation.html
  • setup.py of Pillow runs _find_include_file and _find_library_file, so their paths can be wrong for system zlib: https://github.com/python-pillow/Pillow/blob/master/setup.py#L353
• You're right, depends_on "jpeg" would be appropriate

[yous]

I saw python-pillow/Pillow#4974, and it's available from Pillow 8.0.0, but beats 7.9.2 specifies Pillow 7.0.0: https://github.com/elastic/beats/blob/v7.9.2/libbeat/tests/system/requirements.txt

Also, Pillow would be removed from beats 7.10.0: elastic/beats#20407

[fxcoudert]

It says it doesn't actually use Pillow, it's just… there for no reason?

[fxcoudert]

Superseded by #63381