Matplotlib error on testing

[ben18785]

Matplotlib looks like it has now added error checking for the value of alpha (which controls the transparency of plots) as I now get the following types of error when running the unit tests on Github.

ERROR: test_series (test_plot.TestPlot)
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Traceback (most recent call last):
  File "/home/runner/work/pints/pints/pints/tests/test_plot.py", line 416, in test_series
    pints.plot.series(few_samples, self.problem, thinning=1)
  File "/home/runner/work/pints/pints/pints/plot/_series.py", line 110, in series
    plt.plot(times, v, color='#1f77b4', alpha=alpha)
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/pyplot.py", line 2988, in plot
    return gca().plot(
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/axes/_axes.py", line 1605, in plot
    lines = [*self._get_lines(*args, data=data, **kwargs)]
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/axes/_base.py", line 315, in __call__
    yield from self._plot_args(this, kwargs)
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/axes/_base.py", line 539, in _plot_args
    return [l[0] for l in result]
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/axes/_base.py", line 539, in <listcomp>
    return [l[0] for l in result]
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/axes/_base.py", line 532, in <genexpr>
    result = (make_artist(x[:, j % ncx], y[:, j % ncy], kw,
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/axes/_base.py", line 354, in _makeline
    seg = mlines.Line2D(x, y, **kw)
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/lines.py", line 397, in __init__
    self.update(kwargs)
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/artist.py", line 1064, in update
    ret.append(func(v))
  File "/opt/hostedtoolcache/Python/3.8.8/x64/lib/python3.8/site-packages/matplotlib/artist.py", line 965, in set_alpha
    raise ValueError(f'alpha ({alpha}) is outside 0-1 range')
ValueError: alpha (5.0) is outside 0-1 range