BLD: Clean up MANIFEST.in

Add a note about why we
```
graft pandas/_libs/src
graft pandas/_libs/tslibs/src
```
per https://github.com/pandas-dev/pandas/pull/41977/files#r650501025

Move versioneer.py up to the top with the
"top-level" stuff.

Delete a few redundant includes that
are already included from the `graft pandas`
and not excluded anywhere else.

Author: NickCrews

MANIFEST.in

@@ -1,4 +1,5 @@
 include RELEASE.md
+include versioneer.py
 
 graft doc
 prune doc/build
@@ -54,9 +55,6 @@ global-exclude *.pxi
 # exclude the whole directory to avoid running related tests in sdist
 prune pandas/tests/io/parser/data
 
-include versioneer.py
-include pandas/_version.py
-include pandas/io/formats/templates/*.tpl
-
+# Selectively re-add *.cxx files that were excluded above
 graft pandas/_libs/src
 graft pandas/_libs/tslibs/src