Fix python version detection in theme bobthefish

Relax the python version matching, so `python` that symlinks to
either major versions (e.g. 2, 3) or minor versions (e.g. 2.7, 3.4)
can be properly detected.

Note that currently both pypy and pypy3 have same display glyph.

fish_prompt.fish

@@ -327,11 +327,11 @@ end
 function __bobthefish_virtualenv_python_version -d 'Get current python version'
   set -l python_version (readlink (which python))
   switch "$python_version"
-    case python2
+    case 'python2*'
       echo $__bobthefish_superscript_glyph[2]
-    case python3
+    case 'python3*'
       echo $__bobthefish_superscript_glyph[3]
-    case pypy
+    case 'pypy*'
       echo $__bobthefish_pypy_glyph
   end
 end