feat: add features.version value (#2357)

This adds a `features.version` value that is populated by git when `git
archive` is run. This means it will be expanded when our release action
runs to the tag name. Otherwise, it isn't expanded. When it isn't
expanded, the value is set to the empty string.

.gitattributes

diff --git a/.gitattributes b/.gitattributes
index e4e5d4b..eae260e 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1 +1,2 @@
+python/features.bzl export-subst
 tools/publish/*.txt linguist-generated=true

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a1a2edc..e85cfb4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -47,6 +47,7 @@
   `requirements_linux.txt`, `requirements_windows.txt` for each respective OS
   and one extra file `requirements_universal.txt` if you prefer a single file.
   The `requirements.txt` file may be removed in the future.
+* The rules_python version is now reported in `//python/features.bzl#features.version`
 
 {#v0-0-0-removed}
 ### Removed

python/features.bzl

diff --git a/python/features.bzl b/python/features.bzl
index 3a10532..90a1121 100644
--- a/python/features.bzl
+++ b/python/features.bzl
@@ -13,6 +13,11 @@
 # limitations under the License.
 """Allows detecting of rules_python features that aren't easily detected."""
 
+# This is a magic string expanded by `git archive`, as set by `.gitattributes`
+# See https://git-scm.com/docs/git-archive/2.29.0#Documentation/git-archive.txt-export-subst
+_VERSION_PRIVATE = "$Format:%(describe:tags=true)$"
+
 features = struct(
+    version = _VERSION_PRIVATE if "$Format" not in _VERSION_PRIVATE else "",
     precompile = True,
 )