gst-env: use Path.open() in get_pkgconfig_variable_from_pcfile()

The pcfile argument passed to get_target_install_filename() is guaranteed to be a Path() object so use the .open() method to open the file instead of the standard open() function.

This makes it possible to run gst-env.py on older systems with pyhton3.5 where the standard open() function cannot handle Path arguments.

The change fixes errors like the following:

-----------------------------------------------------------------------
$ ninja -C build/ devenv
ninja: Entering directory `build/'
[0/1] Running external command devenv
Traceback (most recent call last):
  File "/home/ao2/gst-build/gst-env.py", line 493, in <module>
    env = get_subprocess_env(options, gst_version)
  File "/home/ao2/gst-build/gst-env.py", line 342, in get_subprocess_env
    elif is_gio_module(target, filename, options.builddir):
  File "/home/ao2/gst-build/gst-env.py", line 121, in is_gio_module
    giomoduledir = PurePath(get_pkgconfig_variable(builddir, 'gio-2.0', 'giomoduledir'))
  File "/home/ao2/gst-build/gst-env.py", line 110, in get_pkgconfig_variable
    return get_pkgconfig_variable_from_pcfile(pcfile, varname)
  File "/home/ao2/gst-build/gst-env.py", line 89, in get_pkgconfig_variable_from_pcfile
    with open(pcfile, 'r', encoding='utf-8') as f:
TypeError: invalid file: PosixPath('/home/ao2/gst-build/build/meson-private/gio-2.0.pc')
FAILED: meson-devenv
-----------------------------------------------------------------------

Part-of: !192 (merged)

(cherry picked from commit fa9627eb)