Add LDFLAGS for Xcode SDK zlib on macOS >= 1100 to resolve build issues on Big Sur.

... which is caused by `realpath.dylib` containing illegal UTF-8 byte sequence, and `LC_CTYPE` won't take effect if `LC_ALL` happens to be set to something other than `C`.

This commit fixes issue pyenv/pyenv#1454.

Ref: https://stackoverflow.com/a/23584470

PyPy has now only one development tip, `default`,
others are version-line-specific maintenance branches.
The py3k branch has been closed in 2016.
As such, a separate pypy3-dev entry no longer makes sense.

exclude bin files from coreutils

* Find working PyPy links in downloads.python.org
* Use portable-pypy links from softwareheritage.org

Fix homebrew link in README.

Remove extra whitespace in quoted package_url#checksum string.

Prevent the following error:

  unexpected checksum length: 65 (e2562a8d235adc19be5451c170837f53ef916aec4cd5cd17d9e0ab1f1b875d3f )
  expected 0 (no checksum), 32 (MD5), or 64 (SHA2-256)

This makes it easier to copy and paste commands from the README.

Update 3.9-dev and add 3.10-dev

After the feature freeze, master now points to 3.10.

The test "prefix for system in /" is duplicated in test/prefix.bats.
Both tests are completely identical. This commit removes the
duplication.

It appears that the culprit is this merge from rbenv/master, from 2016:

  cf1beda362

With the current development version of bats, this leads to the
following error when running `make test`:

  Error:
  Duplicate test name(s) in file
  "/home/travis/build/pyenv/pyenv/test/prefix.bats":
  test_prefix_for_system_in_-2f

Note that the development version is what pyenv's Makefile uses. With
the latest release of bats, the duplication only leads to a warning:

  bats warning:
  duplicate test name(s) in /src/test/prefix.bats:
  test_prefix_for_system_in_-2f

* Pin bats to the latest release (1.2.0)

This fixes the following error when running `make test` with bats
installed from its master branch:

  /src/bats/libexec/bats-core/bats-exec-file:
  line 192:
  bats-exec-test:
  command not found

The Makefile currently runs bats from its master branch. This can lead
to errors when bats is broken between releases, as is currently the case
with bats at the following commit:

  bats-core/bats-core@b615ed8f750e45017b1ad070ef893d1e2552633a

Instead, use the latest release of bats, which is 1.2.0 (2020-04-25).

* Pin bats to latest release on Travis CI

* Remove Ruby references from python-build

* restore fix_directory_permissions