ipython_directive: Adjust doc examples for reproducibility.

Before this change, building the documentation twice in a row in an
environment configured for reproducible bulids would result in
discrepancies such as:

   diff -ru /gnu/store/...-python-ipython-documentation-8.2.0/share/doc/python-ipython-documentation-8.2.0/html/sphinxext.html /gnu/store/...-python-ipython-documentation-8.2.0-check/share/doc/python-ipython-documentation-8.2.0/html/sphinxext.html
   --- /gnu/store/...-python-ipython-documentation-8.2.0/share/doc/python-ipython-documentation-8.2.0/html/sphinxext.html       1969-12-31 19:00:01.000000000 -0500
   +++ /gnu/store/...-python-ipython-documentation-8.2.0-check/share/doc/python-ipython-documentation-8.2.0/html/sphinxext.html 1969-12-31 19:00:01.000000000 -0500
   @@ -682,7 +682,7 @@
    <span class="gp">In [2]: </span><span class="kn">import</span> <span class="nn">datetime</span>
    <span class="gp">   ...: </span><span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">()</span>
    <span class="gp">   ...: </span>
   -<span class="gh">Out[2]: </span><span class="go">datetime.datetime(2022, 4, 17, 3, 21, 14, 978155)</span>
   +<span class="gh">Out[2]: </span><span class="go">datetime.datetime(2022, 4, 17, 3, 37, 37, 115081)</span>
    </pre></div>
    </div>
    <p>It supports IPython construct that plain
   @@ -690,7 +690,7 @@
    <div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [3]: </span><span class="kn">import</span> <span class="nn">time</span>

    <span class="gp">In [4]: </span><span class="o">%</span><span class="k">timeit</span> time.sleep(0.05)
   -<span class="go">50.2 ms +- 104 us per loop (mean +- std. dev. of 7 runs, 10 loops each)</span>
   +<span class="go">50.1 ms +- 8.86 us per loop (mean +- std. dev. of 7 runs, 10 loops each)</span>
    </pre></div>
    </div>
    <p>This will also support top-level async when using IPython 7.0+</p>

* IPython/sphinxext/ipython_directive.py: Use a fixed date string in
the datetime example, and replace the %timeit example by %pdoc, whole
output is static.

IPython/sphinxext/ipython_directive.py

@@ -19,7 +19,7 b' Here is an example of how the IPython directive can'
    In [1]: 1+1
 
    In [1]: import datetime
-      ...: datetime.datetime.now()
+      ...: datetime.date.fromisoformat('2022-02-22')
 
 It supports IPython construct that plain
 Python does not understand (like magics):
@@ -28,7 +28,7 b' Python does not understand (like magics):'
 
    In [0]: import time
 
-   In [0]: %timeit time.sleep(0.05)
+   In [0]: %pdoc time.sleep
 
 This will also support top-level async when using IPython 7.0+