gpython: Fix crash when invoked as e.g. ./bin/gpython

There was an assert because sys.path[0] is setup as realpath (real, not
relative), but if exe was given as relative path it was raising like
below:

    gpython/gpython_test.py::test_pymain_run_via_relpath GPython 0.0.7.post1 [gevent 20.9.0] / CPython 3.8.6
    Traceback (most recent call last):
      File "./__init__.py", line 482, in <module>
        main()
      File "./__init__.py", line 366, in main
        pymain(argv, init)
      File "./__init__.py", line 82, in pymain
        raise RuntimeError('pymain: internal error: sys.path[0] was not set by underlying python to dirname(exe):'
    RuntimeError: pymain: internal error: sys.path[0] was not set by underlying python to dirname(exe):

            exe:    ./__init__.py
            sys.path[0]:    /home/kirr/src/tools/go/pygolang-master/gpython
    FAILED

-> Fix it by setting up exe as realpath.

gpython/__init__.py

@@ -50,6 +50,7 @@ _pyopt_long = ('version',)
 # init, if provided, is called after options are parsed, but before interpreter start.
 def pymain(argv, init=None):
     import sys
+    from os.path import dirname, realpath
 
     # sys.executable
     # on windows there are
@@ -57,7 +58,7 @@ def pymain(argv, init=None):
     #   gpython.exe
     #   gpython.manifest
     # and argv[0] is gpython-script.py
-    exe  = argv[0]
+    exe  = realpath(argv[0])
     argv = argv[1:]
     if exe.endswith('-script.py'):
         exe = exe[:-len('-script.py')]
@@ -65,14 +66,11 @@ def pymain(argv, init=None):
     sys._gpy_underlying_executable = sys.executable
     sys.executable  = exe
 
-    import os
-    from os.path import dirname, realpath
-
     # `python /path/to/gpython` adds /path/to to sys.path[0] - remove it.
     # `gpython file` will add path-to-file to sys.path[0] by itself, and
     # /path/to/gpython is unneccessary and would create difference in behaviour
     # in between gpython and python.
-    exedir = exe[:exe.rindex(os.sep)] # dirname, but os.path cannot be imported yet
+    exedir = dirname(exe)
     if sys.path[0] == exedir:
         del sys.path[0]
     else:

gpython/gpython_test.py

@@ -274,6 +274,13 @@ def test_pymain_ver(runtime):
     ret, out, err = _pyrun(['--version'], stdout=PIPE, stderr=PIPE, env=gpyenv(runtime))
     assert (ret, out, b(err)) == (0, b'', b(vok))
 
+# verify that ./bin/gpython runs ok.
+@gpython_only
+def test_pymain_run_via_relpath():
+    argv = ['-c',  'import sys; print(sys.version)']
+    out1 = pyout(                    argv, pyexe=sys.executable)
+    out2 = pyout(['./__init__.py'] + argv, pyexe=sys._gpy_underlying_executable, cwd=here)
+    assert out1 == out2
 
 # verify -X gpython.runtime=...
 @gpython_only