ci: rm workarounds from `install-deps-osx` action

The issue [1] is closed, and it looks like the problem doesn't exist
now. So removing the respective workaround from the action.

Also, removing the workaround with upgrading packages as it is not
needed anymore because now `brew` just gives a warning instead of
failing when the package is already installed.

[1] https://github.com/gevent/gevent/issues/1721

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci

.github/actions/install-deps-osx/action.yml

@@ -6,23 +6,16 @@ runs:
   using: composite
   steps:
     - run: |
-        # FIXME: Temporary pinned python3 to specific version (python@3.8)
-        # to avoid 'gevent' package installation failure described in
-        # gevent/gevent#1721. Change to python3 when the issue is resolved.
-        PKGS='openssl
-              readline
-              curl
-              icu4c
-              libiconv
-              zlib
-              cmake
-              ninja
-              python@3.8
-              autoconf
-              automake
-              libtool'
-
-        # Try to install the packages or upgrade them if the packages are
-        # already installed.
-        brew install --force ${PKGS} || brew upgrade ${PKGS}
+        brew install --force \
+          openssl \
+          readline \
+          curl \
+          icu4c \
+          libiconv \
+          zlib \
+          cmake \
+          ninja \
+          autoconf \
+          automake \
+          libtool
       shell: bash