treewide: codestyle fixes

[jvesely]

Use == to compare strings
Drop unused imports.
Use is or is not when comparing to True or False.
2 blank lines after function/class definition.

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.7-x64):

diff -r docs-base/_downloads/08bdb54817a4817ad8e274272625aa24/Cohen_Huston1994_horse_race.py docs-head/_downloads/08bdb54817a4817ad8e274272625aa24/Cohen_Huston1994_horse_race.py
208a209
> 
242a244
> 

...

See CI logs for the full diff.

[lgtm-com]

This pull request fixes 45 alerts when merging ab86c16 into aea0621 - view on LGTM.com

fixed alerts:

• 33 for Unused import
• 12 for Comparison using is when operands support `__eq__`

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.7-x64):

diff -r docs-base/_downloads/08bdb54817a4817ad8e274272625aa24/Cohen_Huston1994_horse_race.py docs-head/_downloads/08bdb54817a4817ad8e274272625aa24/Cohen_Huston1994_horse_race.py
208a209
> 
242a244
> 

...

See CI logs for the full diff.

[lgtm-com]

This pull request fixes 45 alerts when merging 42c1e7e into aea0621 - view on LGTM.com

fixed alerts:

• 33 for Unused import
• 12 for Comparison using is when operands support `__eq__`

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.7-x64):

diff -r docs-base/Composition.html docs-head/Composition.html
4027c4027
< <dd class="field-even"><p><p><strong>2d list of values of OUTPUT Nodes at end of last trial</strong> – each item in the list is the <a class="reference internal" href="Mechanism.html#psyneulink.core.components.mechanisms.mechanism.Mechanism_Base.output_values" title="psyneulink.core.components.mechanisms.mechanism.Mechanism_Base.output_values"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">output_values</span></code></a> for an <a class="reference internal" href="#psyneulink.core.compositions.composition.NodeRole.OUTPUT" title="psyneulink.core.compositions.composition.NodeRole.OUTPUT"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">OUTPUT</span></code></a> <a class="reference internal" href="#composition-nodes"><span class="std std-ref">Node</span></a> of the Composition, listed in the order listed in <a class="reference internal" href="#psyneulink.core.compositions.composition.Composition.get_nodes_by_role" title="psyneulink.core.compositions.composition.Composition.get_nodes_by_role"><code class="xref any py py-meth docutils literal notranslate"><span class="pre">get_nodes_by_role</span></code></a>(<a class="reference internal" href="#psyneulink.core.compositions.composition.NodeRole.OUTPUT" title="psyneulink.core.compositions.composition.NodeRole.OUTPUT"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">NodeRole.OUTPUT</span></code></a>).</p>
---
> <dd class="field-even"><p><p><strong>2d list of values of OUTPUT Nodes at end of last trial</strong> – each item in the list is the <a class="reference internal" href="Mechanism.html#psyneulink.core.components.mechanisms.mechanism.Mechanism_Base.output_values" title="psyneulink.core.components.mechanisms.mechanism.Mechanism_Base.output_values"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">output_values</span></code></a> for an <a class="reference internal" href="#psyneulink.core.compositions.composition.NodeRole.OUTPUT" title="psyneulink.core.compositions.composition.NodeRole.OUTPUT"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">OUTPUT</span></code></a> <a class="reference internal" href="#composition-nodes"><span class="std std-ref">Node</span></a> of the Composition, listed in the order listed in <code class="xref any docutils literal notranslate"><span class="pre">get_nodes_by_role</span></code>).</p>
diff -r docs-base/Pathway.html docs-head/Pathway.html
292c292
< <li><p><strong>Pathway</strong> object or constructor: Pathway(pathway=<a class="reference internal" href="#pathway-specification"><span class="std std-ref">Pathway specification</span></a>,…).</p></li>
---
> <li><p><strong>Pathway</strong> object or constructor: Pathway(pathway= <a class="reference internal" href="#pathway-specification"><span class="std std-ref">Pathway specification</span></a>,…).</p></li>
diff -r docs-base/_downloads/08bdb54817a4817ad8e274272625aa24/Cohen_Huston1994_horse_race.py docs-head/_downloads/08bdb54817a4817ad8e274272625aa24/Cohen_Huston1994_horse_race.py
208a209
> 
242a244
> 

...

See CI logs for the full diff.

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.7-x64):

diff -r docs-base/_downloads/08bdb54817a4817ad8e274272625aa24/Cohen_Huston1994_horse_race.py docs-head/_downloads/08bdb54817a4817ad8e274272625aa24/Cohen_Huston1994_horse_race.py
208a209
> 
242a244
> 

...

See CI logs for the full diff.