Deprecate --py

[AndydeCleyre]

I should probably drop the pypy special treatment, and just have one flag (no arguments to it) that is equivalent to --py current.

Need to name it well, too. Suggestions welcome!

Quick check of what functions would be affected:

.zpy_ui_envin () {  # [--py pypy|current] [<reqs-txt>...]
.zpy_ui_activate () {  # [--py pypy|current] [-i|<proj-dir>]
.zpy_ui_a8 () {  # [--py pypy|current] [-i|<proj-dir>]
.zpy_ui_vpyshebang () {  # [--py pypy|current] <script>...
.zpy_ui_vrun () {  # [--py pypy|current] [--cd] [--activate] <proj-dir> <cmd> [<cmd-arg>...]
.zpy_ui_vpy () {  # [--py pypy|current] [--activate] <script> [<script-arg>...]
.zpy_ui_vlauncher () {  # [--link-only] [--py pypy|current] <proj-dir> <cmd> <launcher-dest>
.zpy_pipcheckoldcells () {  # [--py pypy|current] <proj-dir>
.zpy_ui_pipcheckold () {  # [--py pypy|current] [--all|-i|<proj-dir>...]
.zpy_pipup () {  # [--py pypy|current] [--faildir <faildir>] [--only-sync-if-changed] <proj-dir>
.zpy_ui_pipup () {  # [--py pypy|current] [--only-sync-if-changed] [--all|-i|<proj-dir>...]
.zpy_vpy2json () {  # [--py pypy|current] <jsonfile> <keycrumb>...
.zpy_ui_vpysublp () {  # [--py pypy|current]
.zpy_ui_vpyvscode () {  # [--py pypy|current]
.zpy_ui_vpypyright () {  # [--py pypy|current]

[AndydeCleyre]

Also: #28