Fix encoding on Windows

[hugovk]

Fixes #19, same thing as tkrajina/gpxpy#178 (comment).

This also adds an emoji to one of the test files, which before the fix fails on the CI on Windows (but not macOS or Linux): https://github.com/hugovk/strava_py/actions/runs/2987678688

After the fix, all pass.

---

Also use a "context manager" to open the file, instead of:

activity = gpxpy.parse(open(gpxfile, encoding="utf-8"))

Do this:

    with open(gpxfile, encoding="utf-8") as f:
        activity = gpxpy.parse(f)

A context manager takes care of closing file handles in case of errors, and is equivalent to

f = open(gpxfile, encoding="utf-8")
try:
    activity = gpxpy.parse(f)
finally:
    file.close()  # this runs no matter what happens

More info: https://towardsdatascience.com/why-you-should-use-context-managers-in-python-4f10fe231206