Change #!/usr/bin/python to #!/usr/bin/env python (prometheus#945)

Updated #!/usr/bin/python to #!/usr/bin/env python

Signed-off-by: Kishan Verma <kishanverma2312@gmail.com>
Co-authored-by: Kishan Verma <kishanverma2312@gmail.com>

Author: kishan-23

prometheus_client/__init__.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 from . import (
     exposition, gc_collector, metrics, metrics_core, platform_collector,

prometheus_client/openmetrics/exposition.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 
 from ..utils import floatToGoString

prometheus_client/openmetrics/parser.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 
 import io as StringIO