add optional dependencies to PyPI description

Description

With the change to lalpulsar ephemerides, I think we need to make this clear to users. The lalinference and test options were also difficult to discover so far.

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

@duncanmmacleod @leo-singer @karl-wette @adam-mercer please have a look at the wording and maybe syntax too.

wheel/setup.py.in

@@ -19,6 +19,17 @@ Installing LALSuite from the Python Package Index requires pip >= 19.3.
 To install, simply run::
 
     $ pip install lalsuite
+
+Optional dependencies, which can be installed as ``pip install lalsuite[option]``:
+
+* ``lalinference`` (pulls in
+  `gwdatafind <https://pypi.org/project/gwdatafind/>`
+   and `gwpy <https://pypi.org/project/gwpy/>`)
+* ``lalpulsar`` (pulls in
+  `solar_system_ephemerides <https://pypi.org/project/solar-system-ephemerides/>`
+  to provide ephemeris files which, until LALSuite 7.15, were still included in the main package)
+* ``test`` (pulls in
+  `pytest <https://pypi.org/project/pytest/>`)
 """
 
 import os